Xenobiotic compound modulated expression systems and uses therefor

ABSTRACT

A novel nuclear receptor, termed the steroid and xenobiotic receptor (SXR), a broad-specificity sensing receptor that is a novel branch of the nuclear receptor superfamily, has been discovered. SXR forms a heterodimer with RXR that can bind to and induce transcription from response elements present in steroid-inducible cytochrome P450 genes in response to hundreds of natural and synthetic compounds with biological activity, including therapeutic steroids as well as dietary steroids and lipids. Instead of hundreds of receptors, one for each inducing compound, the invention SXR receptors monitor aggregate levels of inducers to trigger production of metabolizing enzymes in a coordinated metabolic pathway. Agonists and antagonists of SXR are administered to subjects to achieve a variety of therapeutic goals dependent upon modulating metabolism of one or more endogenous steroids or xenobiotics to establish homeostasis. An assay is provided for identifying steroid drugs that are likely to cause drug interaction if administered to a subject in therapeutic amounts. Transgenic animals are also provided which express human SXR, thereby serving as useful models for human response to various agents which potentially impact P450-dependent metabolic processes. Also provided are expression systems and expression vectors having SXR receptors and the like operably linked to target genes of interest.

RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. application Ser. No.09/458,366, filed Dec. 9, 1999, which is, in turn, aContinuation-in-Part of U.S. Ser. No. 09/227,718, filed Jan. 8, 1999,which is, in turn, a Continuation-in-Part of U.S. Ser. No. 09/005,286,filed Jan. 9, 1998, the entire contents of each of which are herebyincorporated by reference herein.

ACKNOWLEDGMENT

This invention was made with United States Government support underGrant No. DK57978, awarded by the National Institutes of Health. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to intracellular receptors, nucleic acidsencoding same, and uses therefor. In a particular aspect, the presentinvention relates to methods for the modulation of physiologicalresponse to elevated levels of steroid and/or xenobiotic compounds. Inanother aspect, the present invention relates to controllable expressionsystems for the modulation and/or production of a gene product and/ortarget protein.

BACKGROUND OF THE INVENTION

Nuclear receptors constitute a large superfamily of ligand-dependent andsequence-specific transcription factors. Members of this familyinfluence transcription either directly, through specific binding to thepromoters of target genes (see Evans, in Science 240:889-895 (1988)), orindirectly, via protein-protein interactions with other transcriptionfactors (see, for example, Jonat et al., in Cell 62:1189-1204 (1990),Schuele et al., in Cell 62:1217-1226 (1990), and Yang-Yen et al., inCell 62:1205-1215 (1990)). The nuclear receptor superfamily (also knownin the art as the “steroid/thyroid hormone receptor superfamily”)includes receptors for a variety of hydrophobic ligands, includingcortisol, aldosterone, estrogen, progesterone, testosterone, vitamin D3,thyroid hormone and retinoic acid, as well as a number of receptor-likemolecules, termed “orphan receptors” for which the ligands remainunknown (see Evans, 1988, supra). These receptors all share a commonstructure indicative of divergence from an ancestral archetype.

Lipophilic hormones such as steroids, retinoic acid, thyroid hormone,and vitamin D3 control broad aspects of animal growth, development, andadult organ physiology. The effects of these hormones are mediated by alarge superfamily of intracellular receptors that function asligand-dependent and sequence-specific transcription factors. Thenon-steroidal nuclear receptors for thyroid hormone (TR), vitamin D3(VDR), all-trans retinoic acid (RAR), and fatty acids and eicosanoids(PPAR) form heterodimers with the 9-cis retinoic acid receptor (RXR)that bind bipartite hormone-response elements (HREs) composed ofdirectly repeated half sites related to the sequence AGGTCA (Mangelsdorfand Evans, Cell 83: 841-850, 1995). In contrast, the steroid receptorsfunction as homodimers and bind to palindromic target sequences spacedby three nucleotides (Beato et al., Cell 83: 851-857, 1995). In additionto the known receptors, a large group of structurally-related “orphan”nuclear receptors has been described which possess obvious DNA andligand binding domains, but lack identified ligands (Mangelsdorf et al.,Cell 83:835-839, 1995; Enmark and Gustafsson, Mol. Endocrinol. 10:1293,1996); and O'Malley and Conneely, Mol. Endocrinol. 6:1359, 1992)). Eachhas the potential to regulate a distinct endocrine signaling pathway.

It is widely viewed that the hormone response is a consequence of therelease, from an endocrine gland, of a ligand that circulates throughthe blood, and coordinately regulates responses in target tissues byacting through specific nuclear receptors. Hormone responsiveness isdependent on the ability to rapidly clear ligand from the blood and thebody so that, in the absence of a stimulus, target tissues return to aground state. Hormonal homeostasis is thus achieved by the coordinatedrelease and degradation of bioactive hormones. Steroid hormones andtheir many metabolites are primarily inactivated by reduction andoxidation in the liver. Since hundreds of adrenal steroids have beenidentified (e.g., dozens of each of the sex steroids (androgens,estrogens and progestins), 25-35 vitamin D metabolites, and likelyhundreds of fatty acids, eicosanoids, hydroxyfats and related bioactivelipids), the problem of efficient ligand elimination is critical tophysiologic homeostasis. In addition to the existence of a myriad ofendogenous hormones, a similar diversity of ingested plant and animalsteroids and bioactive xenobiotic compounds must also be degraded. Suchcompounds often are lipophilic and may accumulate to toxic levels unlessthey are metabolized to water-soluble products that can be readilyexcreted. Therefore, the efficient detoxification of harmful xenobioticsis essential to the survival of all organisms.

Selye first introduced the concept that exogenous steroids andpharmacologic substances may function to modulate the expression ofenzymes that would protect against subsequent exposure to toxicxenobiotic substances (H. Selye, J. Pharm. Sci. 60:1-28, 1971). Thesecompounds, which Selye called “catatoxic steroids,” are typified by thesynthetic glucocorticoid antagonist, pregnenolone-16-carbonitrile (PCN).PCN, and a variety of xenobiotic steroids, induce the proliferation ofhepatic endoplasmic reticulum and the expression of cytochrome P450genes (Burger et al., Proc. Natl. Acad. Sci. (USA) 89:2145-2149, 1992;Gonzalez et al., Mol. Cell. Biol. 6:2969-2976, 1986; and Schuetz andGuzelian, J. Biol. Chem. 259:2007-2012, 1984). Cytochrome P450 (CYP)enzyme(s), present in the endoplasmic reticulum of livers, oftencatalyze the initial step in such detoxification pathways. P450's arecrucial for the detoxification of most xenobiotics, including variousenvironmental pollutants, procarcinogens, and drugs (for review seeDenison M S and Whitlock Jr, J. Biol. Chem. 270:18175-18178, 1995). Inaddition, CYPs are also responsible for the reduction and oxidation ofsteroid hormones and their many metabolites.

One consequence of PCN treatment is the induction of nonspecific“protection” against subsequent exposure to such diverse xenobiotics asdigitoxin, indomethacin, barbiturates, and steroids (Selye, supra,1971). Furthermore, it is known that a variety of such compounds canactivate P450 genes responsible for their detoxification or degradation(Fernandez-Salguero and Gonzalez, Pharmacogenetics 5:S123-128, 1995;Denison and Whitlock, supra 1995; O. Hankinson, Ann. Rev. Pharmacol.Toxicol. 35:307-340, 1995; and Rendic and Di Carlo, Drug Metab. Rev.29:413-580, 1997). P450's constitute a superfamily; each form possessesan overlapping but distinct substrate specificity. Some P450 genes areexpressed constitutively, while others, particularly those involved inxenobiotic metabolism, are inducible. In many cases, inducers are alsosubstrates for the induced enzymes, therefore, P450 activities typicallyremain elevated only as needed. Among the CYP gene family members, theCYP3A isoenzyme is of particular significance from a medicalperspective. The human CYP3A4 enzyme is involved in the metabolism of alarge number of clinical drugs including antibiotics, antimycotics,glucocorticoids, and the statin class of the HMG-CoA reductaseinhibitors (Maurel P, Ioannides C Ed. (CRC Press, Boca Raton, Fla.) pp.241-270, 1996). Indeed, the drug-induced CYP3A4 activation constitutesthe molecular basis for a number of important clinically known drug-druginteractions. CYP3A23 and CYP3A11 are rodent homologues of CYP3A4 in ratand mouse, respectively. Indeed, purified CYP3A11 (P450MDX-B) exhibitedcomparable activity to CYP3A1 (another rat CYP3A homologue, Halvorson,et al., Arch. Biochem. Biophys. 277:166-180, 1990) and CYP3A4 (Yamazaki,and Shimada, Arch. Biochem. Biophys., 346:161-169, 1996) fortestosterone 6β-hydroxylation, which is thought to be one of thespecific reactions for the CYP3A enzyme in rodents and primates(Matsunaga et al., 1998). The regions of the 5′ regulatory sequences ofCYP3A23 and CYP3A11 share high homology, including multiple putativeresponse elements (Toide et al., Arc. Biochem. Biophy. 338:43-49, 1997),indicating similar transcriptional regulatory mechanisms among theserodent CYP3A genes.

Although there are substantial structural and catalytic similaritiesamong the various members of the CYP3A family across species lines,important differences exist in regulatory control of these genes (forreview, see Gonzalez F J., Pharmacol. Ther. 45:1-138, 1990, and Nelson DR., Arch. Biochem. Biophy., 369:1-10, 1999). For example, a cleardiscrepancy between human and rodents is that the antibiotic RIF inducesCYP3A4 in human liver (Watkins et al., N Engl J Med 338:916-917, 1985)but does not induce CYP3A23 in rats (Wrighton et al, Mol Parmacol28:312-321, 1985) and CYP3A11 in mice (Schuetz et al., Proc Natl AcadSci USA 93:4001-4005, 1996), respectively. On the other hand, theanti-glucocorticoid PCN, which induces CYP3A23 in rat liver (Wrighton etal, 1985), only weakly induces human CYP3A4 (Schuetz et al., Hepatology18:1254-1262, 1993, Kocarek et al., Drug Metab Dispos 23:415-421, 1995,Blumberg et al, Genes Dev 12:3149-3155, 1998), and does not induceCYP3A6 (Dalet et al., DNA 7: 39-46, 1988), a rabbit homolog with a drugresponse specificity similar to CYP3A4 (Barwick et al, Mol Pharmacol50:10-16, 1996). Given the widespread metabolic importance of CYP3A, itwould be of great clinical benefit to find an appropriate animal modelfor use in developing a better understanding of the regulatory controland inter-individual heterogeneity in liver expression of CYP3A inhumans.

While it appears that catatoxic compounds such as PCN regulate theexpression of cytochrome P450s and other detoxifying enzymes, two linesof evidence argue that such regulation is independent of the classicalsteroid receptors. First, many of the most potent compounds (e.g., PCN,spironolactone, and cyproterone acetate) have been shown to be steroidreceptor antagonists; whereas others (e.g., dexamethasone) are steroidreceptor agonists (Burger, supra, 1992). Second, the nonspecificprotective response remains after bilateral adrenalectomy (andpresumably in the absence of adrenal steroids), but not after partialhepatectomy (Selye, supra, 1971).

Insight into the mechanism by which PCN exerts its catatoxic effects isprovided by the demonstration that PCN induces the expression of CYP3A1and CYP3A2, two closely related members of the P450 family ofmonooxygenases (see, for example, Elshourbagy and Guzelian in J. Biol.Chem. 255:1279 (1980); Heuman et al., in Mol. Pharmacol. 21:753 (1982);Hardwick et al., in J. Biol. Chem. 258:10182 (1983); Scheutz andGuzelian in J. Biol. Chem. 259:2007 (1984); Scheutz et al., in J. Biol.Chem. 259:1999 (1984); and Gonzalez et al., in J. Biol. Chem. 260:7435(1985)). The CYP3A hemoproteins display broad substrate specificity,hydroxylating a variety of xenobiotics (e.g., cyclosporin, warfarin anderythromycin), as well as endogenous steroids (e.g., cortisol,progesterone, testosterone and DHEA-sulfate. See, for example, Nebertand Gonzalez in Ann. Rev. Biochem. 56:945 (1987) and Juchau in Life Sci.47:2385 (1990)). A PCN response element (which is highly conserved inthe CYP3A2 gene promoter) has since been identified in subsequentstudies with the cloned CYP3A1 gene promoter (see Miyata et al., inArchives Biochem. Biophysics 318:71 (1995) and Quattrochi et al., in J.Biol. Chem. 270:28917 (1995)). This response element comprises a directrepeat of two copies of the nuclear receptor half-site consensussequence AGTTCA.

In addition to inducing CYP3A gene expression, PCN has also been shownto have marked effects on hepatic cholesterol homeostasis. These effectsinclude significant decreases in the levels of HMG-CoA reductase andcholesterol 7a-hydroxylase gene expression, with associated reductionsin sterol biosynthesis and bile acid secretion. PCN has also beenreported to enhance the formation of cholesterol esters and thehypersecretion of cholesterol into the bile. Thus, PCN affects keyaspects of cholesterol metabolism, including its biosynthesis, storageand secretion.

Activation of orphan nuclear receptor(s) by catatoxic steroids providesa possible mechanism for the induction of xenobiotic metabolizingenzymes by compounds that do not activate known steroid receptors.Because such enzymes are activated by high (pharmacological) doses ofxenobiotic and natural steroids, such a “sensor” would be expected to bea broad-specificity, low-affinity receptor. Such receptors could beactivated not only by endogenous steroids and metabolites but also byexogenous compounds such as phytosteroids, xenobiotics and pharmacologicinducers. Indeed, it is known that a variety of such compounds canactivate P450 genes responsible for their detoxification or degradation(see, for example, Fernandez-Salguero and Gonzalez in Pharmacogenetics5:S123 (1995); Denison and Whitlock, Jr., supra, 1995); Hankinson inAnn. Rev. Pharmacol. Toxicol. 35:307 (1995); and Rendic and Di Carlo inDrug Metab. Rev. 29:413 (1997)).

In healthy individuals, steroid levels are tightly regulated, withincreased catabolism of endogenous steroids being compensated by thepituitary releasing an increase of ACTH, which stimulates biosynthesis,and maintenance of plasma steroid levels. The increased catabolism isreflected by elevated urinary levels of steroid metabolites. Indeed, itis already known that treatment with rifampicin increases urinarymetabolites, such as 6β-hydroxycortisol (Ohnhaus et al., Eur. J. Clin.Pharmacol. 36:39-46, 1989; and Watkins et al., J. Clin. Invest.,83:688-697, 1989), and bile acid metabolites, such as 6β-hydroxyhyocholic and 6α-hyodeoxycholic acids (Wietholtz et al., J. Hepatol,24:713-718, 1996), while the plasma levels of many circulating steroidsrise slightly due to increased synthesis (Lonning et al., J. SteroidBiochem. 33:631-635, 1989; Bammel et al., Eur. J. Clin. Pharmacol,42:641-644, 1992; and Edwards et al., Lancet 2:548-551, 1974).

When synthetic steroids, such as prednisolone (McAllister et al., Br.Med. J. 286:923-925, 1983; and Lee et al., Eur. J. Clin. Pharmaco,45:287-289, 1993) or 17α-ethynylestradiol (F. P. Guengerich, Life Sci.,47:1981-1988, 1990) are administered together with rifampicin, plasmalevels are rapidly decreased due to enhanced urinary clearance. In somepatients undergoing rifampicin therapy for tuberculosis, the increase inurinary steroid levels has led to misdiagnosis of Cushing's syndrome(Kyriazopoulou and Vagenakis, J. Clin. Endocrinol. Metab., 75:315-317,1992; Zawawi et al., Ir. J. Med. Sci., 165:300-302, 1996; and Terzolo etal., Horm. Metab. Res., 27:148-150, 1995). In these patients, steroidproduction and clearance normalized when rifampicin was withdrawn. Inpatients with Addison's disease, who mostly lack the ability tosynthesize adrenal steroids, rifampicin treatment leads to rapiddepletion of endogenous and administered steroids. These documentedclinical situations confirm that induction of CYP3A4 causes increasedsteroid catabolism (Kyriazopoulou et al., J. Clin. Endocrinol. Metab.59:1204-1206, 1984; and Edwards, supra, 1974). However, the art issilent regarding the mechanism by which steroid metabolism is regulatedin the body.

Although therapeutically administered steroids are beneficial inachieving therapeutic goals, such compounds can, in some cases, increasethe overall level of steroids and xenobiotics above physiologicallycompatible levels in the subjects to whom they are administered. Inother cases, the increased level of steroids and/or xenobiotics maylinger in the body longer than is therapeutically required. In addition,some subjects are treated with combinations of steroids and xenobioticsthat may be administered separately to treat different conditions, butwhich, in combination, have an additive, or even synergistic, effectknown as a drug interaction. In such cases, the patient may be unawarewhen a physiologically incompatible level of steroids and xenobioticshas been reached, or when an otherwise therapeutic amount of a steroidbecomes potentially dangerous due to combined effects of separatelyadministered drugs.

Thiazolidinediones (TZDs) are a new class of oral antidiabetic agents,and have been identified to be the synthetic ligands for peroxisomeproliferator-activated gamma (PPARγ) (for reviews, see Spiegelman, 1998,and Wilson and Wahli, 1997). Troglitazone is the first TZD introducedfor clinical use. Although troglitazone is effective in reducinghyperglycemia, concern has been raised by several reports of severehepatic dysfunction leading to hepatic failure in patients receiving thedrug (Neuschwander-Tetri et al, 1998, Shibuya et al., 1998, and for areview, see Watkins and Whitcomb, 1998). The mechanism of the livertoxicity by TZDs remains largely unknown.

Accordingly, there is still a need in the art for the identification andcharacterization of broad specificity, low affinity receptors thatparticipate in the mediation of the physiological effect(s) of steroidsand xenobiotics, particularly when combinations of such compoundsdisrupt homeostasis or cause drug interaction.

SUMMARY OF THE INVENTION

In accordance with the present invention a novel class of human orphannuclear receptor the steroid and xenobiotic receptor (SXR) has beenisolated and characterized. SXR is expressed almost exclusively in theliver, the primary site of xenobiotic and steroid catabolism. Unlikeclassical steroid receptors, SXR heterodimerizes with RXR and binds todirectly repeated sequences related to the half-site, AGTTCA. SXR canactivate transcription through response elements found in some steroidinducible P450 genes in response to an enormous variety of natural andsynthetic steroid hormones, including antagonists such as PCN, as wellas xenobiotic drugs, and bioactive dietary compounds, such asphytoestrogens. The ability of SXR to regulate expression of catabolicenzymes in response to this diversity of steroid and/or xenobioticcompounds provides a novel mechanism for direct regulation of metabolismso as to achieve physiologic homeostasis with respect to such steroidand/or xenobiotic compounds—ideal properties for a “steroid sensingreceptor” which mediates the physiological effect(s) of hormones. SXRrepresents the first new class of steroid receptors described since theidentification of the mineralocorticoid receptor ten years ago.

In accordance with a particular aspect of the present invention, thereare also provided nucleic acid sequences encoding the above-identifiedreceptors, as well as constructs and cells containing the same,expression systems comprising such receptors and/or the correspondingresponse elements, therapeutic expression systems comprising suchreceptors for induction and/or modulation of target proteins and probesderived therefrom. There are also provided transgenic animals expressinghuman SXR. Furthermore, it has also been discovered that a wide varietyof substrates modulate the transcription activating effects of inventionreceptors.

An important requirement for physiologic homeostasis is the removal anddetoxification of various endogenous hormones and xenobiotic compoundswith biological activity. Much of the detoxification is performed bycytochrome P450 enzymes, many of which have broad substrate specificityand are inducible by a bewildering array of compounds, includingsteroids. The ingestion of dietary steroids and lipids induces the sameenzymes and, thus, must be integrated into a coordinated metabolicpathway. Instead of possessing hundreds of receptors, one for eachinducing compound, a class of broad-specificity, low-affinity nuclearreceptors has been discovered that monitor total steroid levels andinduce the expression of genes encoding xenobiotic metabolizing enzymes.SXR, which is a member of a novel branch of the nuclear receptorsuperfamily, forms part of a steroid sensor mechanism for removal ofelevated levels of steroids and/or xenobiotic compounds from circulationvia broad-specificity, low-affinity receptors that represent a novelbranch of the nuclear receptor superfamily.

Several lines of evidence suggest SXR functions as a sensor forxenobiotic compounds and/or steroids, acting as a feedback mechanism inthe liver to regulate the expression of CYP genes: (1) SXR is expressedat high levels in liver and small intestine, two key tissues for steroidand xenobiotic catabolism; (2) Putative SXR response elements, invertedrepeat-6 (IR-6) and direct repeat-3 (DR-3), are found in the genesencoding and/or effected by catabolic enzymes expressed in thesetissues, such as those of the CYP3A4 and CYP3A23, as well as those ofP450 oxidoreductase CYP2A, CYP2C, CYP2E, and glucouronosyl transferase,all known to be involved in steroid and xenobiotic catabolism (for areview, see Gonzalez, F. J., Trends. Pharmacol Sci., 13:346-352, 1992);(3) Compounds known to induce catabolic enzymes such as RIF, nifedipine,steroid agonists and antagonists such as estrogen and tamoxifen, andbioactive dietary compounds such as phytoestrogens, activate a syntheticreporter gene containing these response elements; (4) Some of thesecompounds when partially metabolized (reduced), but still retainingbiological activity, are activators of SXR but not classic steroidreceptors. The recently isolated PXR is the rodent homolog of SXR.Sequence analysis reveals that SXR and PXR share only about 75% aminoacid sequence identity in the ligand binding domain (LDB), in contrastto 95% identity between their DNA binding domains (DBDs) (Blumberg etal., supra, 1998). Comparison of SXR with PXR reveals marked differencein their activation by certain drugs, which may account in part for thespecies-specific effects of compounds on CYP3A gene expression.

In accordance with the present invention, it has been demonstrated thatintroduction of human SXR into rodent hepatocytes or into the liver oftransgenic mice is sufficient to render a human-like profile of CYP3Agene induction by certain drugs such as RIF. For example, constitutiveactivation of SXR and the resulting upregulation of CYP3A gene causesliver toxicity in transgenic mice. In addition, it has been shown thattwo TZDs, troglitazone and ciglitazone, activate CYP3A gene expressionvia SXR, but not PXR, both in cell culture and in transgenic mice. TheSXR-mediated CYP3A gene activation by TZDs, together with thedemonstrated liver toxicity caused by constitutive upregulation of CYP3Ain mouse, provides a potential mechanism for the known liver toxicity bycertain TZDs.

In accordance with another aspect of the presenti invenion there isprovided an expression system that can be efficiently regulated by oneor more FDA-approved drugs. The expression system utilizes endogenousSXR or PXR receptors to regulate target genes in response to exogenouslyadministered activators, such as approved FDA drugs, nutritionalsupplements, and the like. In a particular aspect of the invenion, onecan exploit variations in the ligand binding domain of SXR and/or PXR toenhance or alter affinity for specific drugs. The variations can beeither natural or directed variations and/or mutations to the bindingdomain. In a non-limiting example, a rodent xenobiotic receptor, forexample PXR, fails to respond to administered rifampicin. In contrast, ahuman xenobiotic receptor, for example SXR, fails to respond to therodent inducer xenobiotic molecule PCN. Thus, the rodent receptorsequence, when introduced into an expression vector and placed intomammalian cells having human protein components can be induced,activated and/or controlled by the addition of rodent inducer xenobioticmolecule PCN. This type of system can be implemented by using anexpression vector comprising the human SXR sequence in an expressionvector introduced into rodent cells.

The expression system of the invention can be used in either a cellculture to create expression vector based systems having target genes ofinterest linked to xenobiotic receptor response elements in an operable,or activatable manner, allowing the introduction of a xenobioticcompound and/or excess xenobiotic receptors to cause production of thetarget gene of interest, for example a cytokine (G-CSF), a hormone(e.g., insulin), a blood component (e.g., factor IX), or a toxic protein(triplet repeat or caspace). The expression system also allowscontrolled induction of proteins to manipulate cell growth, function ordifferentiation, when specific target genes/proteins are part of theexpression system. In vivo uses can include expression in transgenicanimals, where it can be important or valuable to create an induciblesystem to serve as a model representing the onset of a human disease orto monitor the consequence of the induction of a particular protein ofpotential therapeutic interest. In addition, the expression system ofthe present invention can be used to create controlled ablation ofcertain target tissues with potential advantages for studying both thedevelopment of the tissue as well as activating or enhancing stem cellproduction. The expression system can also be useful to employ in othereukaryotic cells such as in a yeast-based expression vector system or,as one of skill in the art would recognize, in other eukaryotic cellssuch as in plants for agricultural based use. For example, in plantsexisting pesticides can be identified that would activate expressionsystems comprising proteins of interest operably linked to xenobioticreceptors as described herein to induce and promote production of a geneof interest.

The present expression system can also be employed in the treatment ofhuman disease, as a therapeutic expression system. The expression systemcan be delivered by way of a retroviral-based vector or other genetransfer methodology which is known in the art, where the switch can beintroduced into cells in vivo or to explanted cells which afterinfection or introduction of the expression system, would bere-introduced back into the host, subject and/or donor. Thus, forexample, stem cells could be harvested from bone marrow, infected invitro with high titer virus comprising the expression system of theinvention and then the harvested cells are returned to the host, subjectand/or donor providing a system in which a particular target protein cannow be readily induced to target cells for example orally by taking apill of antibiotic rifampicin, or by suitable administration to cells ofany compound recognized by the xenobiotic receptor as a ligand.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 collectively illustrates that SXR is a novel orphan nuclearreceptor.

FIG. 1A shows the sequence of the longest SXR cDNA clone (SEQ ID NO: 1)and a corresponding encoded protein (amino acids 41-434 of SEQ ID NO:2). The DNA binding domain (amino acids 41-107) is shown in bold, andupstream termination codons in frame with the putative initiator leucineare indicated by asterisks. That this Leu can function as an initiatorwas demonstrated by SDS-PAGE analysis of labeled proteins produced fromin vitro transcribed, translated cDNAs. The unmodified cDNAs yielded atranslation product indistinguishable in size from that produced whenthe leucine was changed to methionine, albeit not nearly as efficient.

FIG. 1C presents a schematic comparison between SXR and other members ofthe steroid hormone receptor super family such as e.g., RXR partners,the Xenopus benzoate X receptor (xBXR), the human vitamin D3 receptor(hVDR), the human constitutively active receptor-alpha (hCARα), the ratfarnesoid X receptor (rFXR), the human peroxisome proliferator activatedreceptor alpha (hPPARα), the human liver-derived receptor X (LXRα), thehuman retinoic acid receptor alpha-1 (hRARα-1), the human thyroidhormone receptor beta (hTRβ), the human retinoid X receptor alpha (RXRα)and the human glucocorticoid receptor alpha (hGRα)). Ligand-bindingdomain boundaries follow those for the canonical nuclear receptorligand-binding domain (Wurtz et al., Nature Struct. Biol. 3:87-94,1996). Similarity between SXR and other receptors is expressed aspercent amino acid identity (indicated in Arabic numerals above eachclone). Amino acid residues in the sequences were aligned using theprogram GAP (Devereaux et al., Nucl. Acids Res. 12:387-395, 1984).DNA=DNA binding domain and LIGAND=ligand binding domain.

FIG. 2 illustrates that SXR is activated by many steroids. Chimericreceptors composed of the GAL4 DNA-binding domain and the SXR-ligandbinding domain were cotransfected into CV-1 cells with the reporter genetk(MH100)₄-luc (Forman et al., Cell 81:541-550, 1995). Results are shownas fold induction over solvent (DMSO) control for 50 μM of steroid andrepresent the averages and standard error from triplicate assays.Neither reporter alone, nor reporter plus GAL4-DBD, was activated by anyof these compounds. Column 1=solvent; column 2=corticosterone; column3=pregnenolone; column 4=dihydrotestosterone (DHT); column5=dehydroepiandrosterone; column 6=progesterone; column 7=dexamethasone;column 7=estradiol; column 8=cortisol; and column 9=cortisone.

FIG. 3 illustrates the ability of steroidal activators to actadditively. Thus, the ability of steroidal activators to act additivelywas tested using full-length SXR and the reporter tk(LXRE)₃-luc (seeWilly et al., in Genes Dev. 9:1033 (1995)). The cocktail contained 10 mMof each steroid for an overall concentration of 100 mM total steroid.The cocktail and its individual components were tested at 100, 10 and 1mM; results are shown in the Figure for 100 mM cocktail and 10 mMaliquots of the component steroids.

FIG. 4 illustrates the broad activator and response element specificityof SXR. Full-length SXR was tested in cotransfection experiments for itsability to activate elements similar to those in FIG. 3 in response to apanel of steroids at 50 mM. DR-1,2 and TREp were only very slightlyactivated, hence results are shown only for corticosterone and PCN. Thedata shown are expressed as mean fold induction over solventcontrol+/−standard error from triplicate assays.

FIG. 5 further illustrates the broad ligand specificity of SXR. Thus, itis seen that reduction of the 4-5 double bond does not inactivatecorticosterone. 6β-hydroxylated, non-reduced, 5α and 5β reduced forms ofcorticosterone were tested for their ability to activate GAL-SXR ontk(MH100)₄-luc and hGRa on MTV-luc at 50 mM. Similar results wereobtained using full-length SXR.

FIGS. 6A-C are a series of illustrations indicating that SXR canactivate responsive elements found in various steroid and xenobioticinducible P450 enzymes.

FIG. 6A (SEQ ID NOs: 3-11, respectively, in order of appearance)presents a schematic comparison of nucleotide sequences encodingresponse elements found in inducible cytochrome P450 enzymes. A databasesearch for repeats of the sequence RGKTCA (SEQ ID NO: 41) was performedand some of the matches for enzymes involved in hepatic steroidhydroxylation are indicated. The standard nomenclature for P450 enzymeshas been utilized. P450R is the single P450 oxidoreductase required forhydroxylation of steroids. UGT1A6 is a rat uridine diphosphate(UDP)-glucuronosyltransferase that conjugates glucuronic acid tohydroxylatecl steroids.

FIG. 6B (SEQ ID NOs: 33-35, respectively, in order of appearance)presents a schematic comparison of conserved glucocorticoid-responseelements found in human CYP3 genes. The region of human CYP3A4 shown isnecessary and sufficient for glucocorticoid and rifampicin induction ofthe full-length promoter. Corresponding regions of CYP3A5 and CYP3A7 areshown (Barwick et al., Mol. Pharmacol. 50:10-16, 1996).

FIG. 6C is a bar graph showing that SXR can activate through inducible,but not uninducible, CYP3 promoter elements. The ability of SXR toactivate tk-CYP3-luc response elements in response to various inducerswas tested. Results are shown for 50 μM compound and represent the meanof triplicate determinations. Refampicin results are shown as open bars;and corticosterone results are shown as filled bars.

FIGS. 7A-C are bar graphs illustrating the ability of a panel ofcompounds to activate a representative of three members of the nuclearreceptor superfamily, human SXR (FIG. 7A); mouse PXR (FIG. 7B); andhuman estrogen receptor alpha (hERα; FIG. 7C). Results are shown for 50μM of compound tested, except that the concentration of tamoxifen was 5μM; and the concentration of dexamethasone (DEX) was 50 μM in FIGS. 7Aand 7B and 5 μM in FIG. 7C. Column 1=solvent; column 2=rifamipicin;column 3=nifedipine; column 4=tamoxifen; column 5=spironolactone; column6=PCN; column 7=DEX; column 8=corticosterone; column 9=cortisone; column10=DHT; column 11=estradiol; column 12=DES; and column 13=coumestrol.

FIG. 7D is a bar graph illustrating that reduction of the 4-5 doublebond in corticosterone does not inactivate the compound as an agonist ofhSXR. 6β-hydroxylated, non-reduced, 5α and 5β reduced forms ofcorticosterone were tested for their ability to activate GAL-hSXR ontk(MH100)₄-luc (lefthand group of 5 columns) and hGRα on MTV-luc at 50μM (righthand group of 5 columns). Similar results were obtained usingfull-length SXR. In each group of columns: column 1=solvent; column2=corticosterone; column 3=5α-tetrahydrocorticosterone; column4=5β-tetrahydrocorticosterone; and column 5=6β-OH-corticosterone.

FIG. 8 provides bar graphs providing information on the induction ofCYP3A genes under control of SXR in a trans-species drug response.

FIG. 8A relates to the mouse CYP3A23 cellular promoter reporter whichwas transfected into primary rat hepatocytes in the absence (open bars)or presence (filled bars) of expression vector for SXR. Cells weresubsequently treated with indicated compounds. Results are shown as foldinduction over solvent (DMSO), and represent the averages and standarderror from triplicate assays. E2, 17β-estradiol; PCN,pregnenolone-16-carbonitrile; 3MC, 3-methylcholanthrene. Theconcentration of compound is 10 μM with exceptions of phenobarbital and3MC (2 mM each). Note the mouse CYP3A23 cellular promoter was activatedin rat hepatocytes by RIF in the presence of SXR.

FIG. 8B describes similar transfection assays as described in FIG. 8Aexcept that the human CYP3A4 cellular promoter reporter was used. Notethe human CYP3A4 cellular promoter was activated by RIF in rathepatocytes in the presence of SXR.

FIG. 8C illustrates that the DR-3 element is essential for SXR-mediatedactivation of CYP3A23, and is interchangeable with the IR-6 element. Thewild type (DR3/WT, SEQ ID NO: 39, filled bars) or mutant forms (DR3/M1,SEQ ID NO: 42, open bars; DR3/M2, SEQ ID NO: 43, stippled bars; andDR3/IR6, SEQ ID NO: 24, hatched bars) of CYP3A23 cellular promoterreporters were transfected into primary rat hepatocytes in the presenceof expression vector for SXR. The ligand treatment and data presentationare the same as in FIG. 8A. RIF, rifampicin; CTZ, clotrimazole. Note thedisruptions of DR-3 element (DR3/M1, and DR3/M2) abrogate the activationof CYP3A23, and the replacement of DR-3 element with IR-6 element(DR3/1R3) rescues the responsiveness.

FIG. 9 presents schematic representations of the Alb-SXR and Alb-VPSXRtransgene constructs. The filled region, stippled region, open region,and the crosshatched region, correspond to the mouse albuminpromoter/enhancer, the xenopus β-globin leader and trailer sequences,the cDNAs of the wild type (SXR) or an activated form of SXR (VPSXR,with the fusion of VP16 activation domain at the 5′ end as depicted),and the SV40 sequence containing the poly (A) processing signal,respectively.

FIG. 10 illustrates the selective activation of SXR by members of thethiazolidinedione family of PPARγ ligands. Thus, the mouse CYP3A23cellular promoter reporter was transfected into primary rat hepatocytesin the absence (open bars) or presence (filled bars) of expressionvector for SXR. Cells were subsequently treated with indicatedcompounds. Results are shown as fold induction over solvent (DMSO), andrepresent the averages and standard error from triplicate assays. Notethe CYP3A23 was activated by synthetic TZDs troglitazone and ciglitazone(10 μM each) in the presence of SXR, whereas the natural PPARγ ligands15d-PGJ2 (3 μM), the synthetic BRL49653 (5 μM), as well as the controlPPARγ specific ligand WY14643 (5 μM), and pan-PPAR ligand LY171883 (30μM), fail to activate CYP3A23. Cotransfection of expression vector formouse PXR did not change the response profile (data not shown).

FIG. 11 illustrates growth retardation in Alb-VPSXR transgenic mice.Thus, males of the Alb-VPSXR (n=9), Alb-SXR (n=7) transgenic mice, ortheir wild type littermates (n=16) were weaned, genotyped and weighed atday 22 after birth, and continue to be weighed every three daysthereafter. The results are presented as the averages and standarderror. Note the consistent lower body weight in Alb-VPSXR mice.

FIG. 12 illustrates the occurance of hepatomegaly in Alb-VPSXRtransgenic mice. Thus, two and half month old males of the Alb-VPSXR(n=8), Alb-SXR (n=8) transgenic mice, or their wild type littermates(n=11) were euthanized. The whole liver, kidney, and spleen (data notshown) were dissected and weighed. The organ weights are presented aspercentages of total body weight. Note the significant increase in liverweight in the Alb-VPSXR mice.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a new class of receptors hasbeen identified that are part of the steroid/thyroid hormone superfamilyof receptors, a representative member of which has been designated SXR(or “steroid X receptor”). Invention receptors are characterized by:

-   -   forming a heterodimer with retinoid X receptor (RXR),    -   binding to a (direct or inverted) repeat response element motif        based on the half site AGTTCA,    -   activating transcription through response elements found in        steroid inducible P450 genes in response to a wide variety of        natural and synthetic steroid hormones, and    -   being prominently expressed in the liver and the intestine.

In one aspect of the present invention, a particular species ofinvention receptor(s) is provided which comprises a protein ofapproximately 464 amino acids (see SEQ ID NO:2), which is most closely,although distantly, related to the Xenopus benzoate X receptor (BXR),the vitamin D3 receptor (VDR) and the constitutively activated receptor(CAR). Also provided herein is a 2068 bp cDNA which encodes an exampleof invention receptors (see SEQ ID NO:1 and FIG. 1A).

In accordance with the present invention, there are also providedmethod(s) for modulating metabolism of one or more steroid and/orxenobiotic compound(s) in a subject in need thereof, comprisingadministering to the subject an effective amount of a modulator of a SXRpolypeptide that activates transcription of an endogenous geneoperatively associated with a steroid and xenobiotic receptor X (SXR)response element.

In one particular aspect of the invention, an expression system isprovided having an SXR response element operably linked to a gene ofinterest and a nuclear receptor that responds to xenobiotic compounds,such as SXR and/or PXR. The expression system is useful for in vitro orin vivo expression of a gene of interest operably linked to saidresponse element, and preferably is used in vivo in a mammal forexample, in a species such as rodent, canine, porcine, equine, andprimate. Preferably the mammal is a human. Within this aspect of theinvention it is preferred that the gene of interest encode for example,secretory proteins that can be released from said cell; enzymes that canmetabolize a substrate from a toxic substance to a non-toxic substance,or from an inactive substance to a useful substance; regulatoryproteins; cell surface receptors; and the like. Useful genes includegenes that encode blood clotting factors such as human factors VIII andIX; genes that encode hormones such as insulin, parathyroid hormone,luteinizing hormone releasing factor (LHRH), alpha and beta seminalinhibins, and human growth hormone; genes that encode proteins such asenzymes, the absence of which leads to the occurrence of an abnormalstate; genes encoding cytokines or lymphokines such as interferons,granulocytic macrophage colony stimulating factor (GM-CSF), colonystimulating factor-1 (CSF-1), tumor necrosis factor (TNF), anderythropoietin (EPO); genes encoding inhibitor substances such asalphal-antitrypsin; genes encoding substances that function as drugs,e.g., genes encoding the diphtheria and cholera toxins; and the like.The xenobiotic compound is generally an exogenous compound that illicitsa response from the SXR, RXR, PXR or like systems, and in non-limitingexamples, such a compound can be digitoxin, indomethacin,pregnelone-16-carbonitrile (PCN), tamoxifen, ralozifene, vitamin K,nifedipine, a barbituate, a steroid, and the like.

In another embodiment, an invention expression system is employed in amethod for administering a target protein to a subject. The inventionmethod can be carried out by administering a gene encoding such a targetprotein and by providing to the subject one or more agonists for aninvention SXR polypeptide to activate transcription of the inventionexpression system having an SXR response element operably linked to agene encoding said target protein, thereby increasing the level ofexpression of the gene and/or target protein of interest in relation toprotein production before administration.

Thus, a method of the invention provides production of a target proteinin a cell and/or subject by administering to a cell and/or subject atleast one xenobiotic compound and a nucleic acid having an SXR and/orPXR response element that is operably linked to at least one geneencoding a target protein. Preferably the cell and/or subject has areceptor which responds to xenobiotic compounds, for example an SXR orPXR. The cell can be a eukaryotic cell such as a plant cell or amammalian cell, most preferably the cell is a mouse or a human cell, andthe subject is a mammal, for example a human or a mouse. Mostpreferably, the expression system is administered to a mammal asdescribed above, more preferably administration is to a mammalianpatient, such as a primate such as a human patient.

In another embodiment, a method is provided to increase production of atarget protein in a cell. This can be accomplished by inducing synthesisof a xenobiotic receptor in a cell that contains an expression vectorencoding a xenobiotic receptor operatively associated with an induciblepromoter and a nucleic acid having an SXR and/or a PXR response elementoperably linked to at least one gene which encodes the target protein.In accordance with this embodiment, production of the target protein canbe induced by the introduction of a xenobiotic compound. Production ofthe target protein can also occur by inducing expression of thexenobiotic receptor. Vectors for use with this embodiment and others,where applicable, are well known to those of skill in the art and can beconfigured and manipulated to contain nucleic acids which serve asinducible promoters, and nucleic acids that can be expressed as fulllength proteins of interest.

In yet another aspect, the invention provides a method for slowingclearance of a therapeutic steroid or xenobiotic from a subject, such asa human or other mammal, which comprises administering to the subject aneffective amount of an antagonist for a SXR polypeptide that activatestranscription of an endogenous gene operatively associated with a SXRresponse element. This aspect of the invention method is useful forcontrolling too rapid clearance of one or more therapeutic steroidsand/or xenobiotics caused by a drug interaction between such compounds.

For example, rifampin (i.e., rifampicin), or an active derivative oranalog thereof, is commonly used to treat tuberculosis. Yet rifampintends to cause hepatic clearance of other therapeutic drugs, such asoral contraceptives (leading to unwanted pregnancy), warfarin (leadingto decreased prothrombin times), cyclosporine and prednisone (leading toorgan rejection or exacerbations of any underlying inflammatorycondition), and verapamil and diltiazem (necessitating increased dosagerequirements). A similar situation develops in treatment of osteoporosiswith the therapeutic steroid Vitamin K. To overcome these problems, inaccordance with the present invention, an effective amount of a SXRpolypeptide antagonist is administered to the patient to slow clearanceof the therapeutic steroids from the subject. Commonly administeredtherapeutic drugs also tend to accumulate or cause a drug interaction incertain individuals leading to an increase in the overall level ofsteroid and xenobiotics above a physiologically suitable level includetamoxifen, ralozifene (e.g., in treatment of breast cancer), vitamin K(e.g., in treatment of osteoporosis), calcium channel blockers, such asnifedipine, and the like.

In yet another aspect, the invention provides a screening assay fordetermining whether test compounds will activate the invention SXRpolypeptide. The assay comprises contacting a host cell line containingan SXR receptor polypeptide, preferably a human or rabbit cell line,with one or more test compound(s) in an appropriate culture medium,wherein the host cell line further contains a reporter vector comprisinga promoter that is operable in the cell line operatively linked to aninvention SXR response element for activation thereof, and DNA encodinga reporter protein operatively linked to the promoter for transcriptionof the DNA. The invention assay further includes determining whether thereporter protein is present (i.e., expressed by the cell line), whereina determination that the reporter is present indicates the test compoundactivates the SXR polypeptide (i.e., an agonist), and a determinationthat the reporter is not present in the assay predicts the test compounddoes not activate the invention SXR polypeptide (i.e., not an agonist).

It has been discovered that compound(s) that activate transcription ofthe DNA contained in the above-described reporter vector are strongagonists of the invention SXR receptor and fall into the category of“steroids and/or xenobiotics” as the term is used herein.

It has further been discovered that compounds determined by the aboveassay to activate transcription of the DNA contained in the abovedescribed reporter vector are likely to become involved in a druginteraction if administered to a subject at a therapeutic level. Moreparticularly, there is a greater than 30% likelihood, for example alikelihood of about 45% to about 90%, or from about 50% to about 70%,that a therapeutic dose of such a compound will cause a drug interactionas described herein, with other steroids and/or xenobiotics, whethersuch compounds are endogenously produced, result from dietary sources,or are therapeutically administered to a subject in treatment of aparticular disease state. Therefore, in one particular aspect, theinvention assay is a method for screening compounds, particularlypotential therapeutic compounds, to determine those with at least a 30%likelihood of becoming involved in an undesirable drug interaction ifadministered to a subject at a therapeutic level. Such a screening assayis a valuable adjunct to any drug development program because it willidentify those drug candidates that must be thoroughly screened in vivoto determine their safety, thereby reducing the cost of drug developmentin general while preventing the possibility that a drug candidate willprove potentially dangerous due to its capacity to cause unhealthyelevation of steroid levels or too rapid clearance of anothertherapeutically administered compound due to a “drug interaction.”

The invention methods are based upon the discovery of a new class ofreceptors identified as part of the steroid/thyroid hormone superfamilyof receptors. The invention receptor, designated herein “the steroid andxenobiotic receptor” (SXR), has been identified as a potential humanhomolog(s) of the Xenopus benzoate ‘X’ receptor, BXR (Blumberg et al.,Genes Dev. 12:1269-1277, 1998). The cDNA encoding one member of the SXRclass (SEQ ID NO:1) predicts a protein of 434 amino acids (SEQ ID NO: 2)(FIG. 1A), which is 73% identical to BXR in the DNA-binding domain (DBD)and 43% identical in the ligand binding domain (LBD) (FIG. 1B). SXR ismost closely related to the recently described pregnane ‘X’ receptor(Kliewer et al., Cell 92:73-82, 1998) (95% identical in the DNA bindingdomain (DBD), and 73% identical in the ligand binding domain (LBD). SXRis more distantly related to the vitamin D3 receptor and the orphanreceptor CAR (Baes et al., Mol. Cell. Biol. 14:544-1551, 1994) (FIG.1B). Other than these receptors, SXR shows no more similarity to othernuclear receptors than the different receptor subfamilies do to eachother (FIG. 1B). It is known that true homologs among nuclear receptorstypically share considerable similarity, especially in the DBD.

SXR can be further characterized as having a DNA binding domain of about67 amino acids with 9 Cys residues (i.e., amino acid residues 41-107, asset forth in SEQ ID NO:2), wherein the SXR DNA binding domain has about73% amino acid identity with the DNA binding domain of the Xenopusbenzoate X receptor. Alternatively, or in addition, SXR can be furthercharacterized as having a ligand binding domain of at least about 294amino acids (i.e., at least amino acid residues 141-434, as set forth inSEQ ID NO:2), wherein said ligand binding domain has about 43% aminoacid identity with the ligand binding domain of the Xenopus benzoate Xreceptor (FIG. 1B).

A presently preferred SXR polypeptide according to the invention is apolypeptide having substantially the same amino acid sequence as shownin SEQ ID NO:2. As employed herein, the phrase “substantially the same,”whether used in reference to the nucleotide sequence of DNA, theribonucleotide sequence of RNA, or the amino acid sequence of protein,refers to sequences that have slight and non-consequential sequencevariations from the actual sequences disclosed herein. Species that aresubstantially the same are considered to be equivalent to the disclosedsequences and as such are within the scope of the appended claims. Inthis regard, “slight and non-consequential sequence variations” meansthat sequences that are substantially the same as the DNA, RNA, orproteins disclosed and/or claimed herein are functionally equivalent tothe sequences disclosed and/or claimed herein. Functionally equivalentsequences will function in substantially the same manner to producesubstantially the same compositions as the nucleic acid and amino acidcompositions disclosed and claimed herein. In particular, functionallyequivalent DNAs encode proteins that are the same as those disclosedherein or proteins that have conservative amino acid variations, such assubstitution of a non-polar residue for another non-polar residue or acharged residue for a similarly charged residue. These changes includethose recognized by those of skill in the art not to substantially alterthe tertiary structure of the protein.

An especially preferred SXR polypeptide according to the inventionmethod is a polypeptide having the same amino acid sequence as shown inSEQ ID NO:2.

Thus, the terms “SXR receptor” and “SXR polypeptide” are interchangeableas used herein and are intended to include functional fragments of theinvention SXR polypeptide(s). Such fragments include peptides having theDNA binding and/or the ligand binding properties of SXR, e.g., the DNAbinding domain thereof (e.g., amino acid residues 71-107 as shown in SEQID NO:2), or the ligand binding domain thereof (e.g., amino acidresidues 141-434 as shown in SEQ ID NO:2).

Modulator(s) useful in the practice of the invention method(s) includeboth agonists and antagonists of the SXR polypeptide. When the modulatoris an agonist, the modulator is characterized as one which activatestranscription of a gene encoding a compound active in catabolism of atherapeutic, endogenous, or dietary steroid, or of certain dietarylipids, which gene is characterized by being associated with a SXRresponse element such that activation of the response element results intranscription of the gene. Generally the gene encodes an enzymeeffective in metabolism of one or more steroids or xenobioticsubstances, such as dietary lipids and phytoestrogens, and also includesa nucleotide sequence that encodes a SXR response element, for example,one having a direct repeat of a suitable half site (the DR half site)separated by a spacing of 3, 4, or 5 nucleotides, or a direct repeat ofa variant thereof.

The response element can also comprise an inverse repeat of a suitablehalf site separated by a 6 nucleotide spacer, or an inverse repeat of avariant thereof, separated by a 6 nucleotide spacer. Those of skill inthe art recognize that the spacing between half sites can vary over aconsiderable range, typically falling in the range of about 0 up to 15nucleotides. When the half sites are oriented as direct repeats, it ispresently preferred that the half sites be separated by a spacer of 3, 4or 5 nucleotides. Those of skill in the art recognize that anycombination of 3, 4 or 5 nucleotides can be used as the spacer. Directrepeat response elements having a spacer of 4 nucleotides (e.g., SEQ IDNOS:6, 7 or 16) are presently preferred. When the half sites areoriented as inverted repeats, it is presently preferred that the halfsites be separated by a spacer of 4, 5 or 6 nucleotides. Those of skillin the art recognize that any combination of 4, 5 or 6 nucleotides canbe used as the spacer.

Half sites contemplated for use herein have the sequence RGBNNM,wherein:

-   -   R is selected from A or G;    -   B is selected from G, C, or T;    -   each N is independently selected from A, T, C, or G; and    -   M is selected from A or C;    -   with the proviso that at least 4 nucleotides of said -RGBNNM-        sequence are identical with the nucleotides at corresponding        positions of the sequence AGTTCA.

Examples of response elements suitable for use in practice of theinvention methods can be selected from the following:

DR-3,4,5=AGGTCANnAGGTCA, wherein n is 3, 4, or 5 (SEQ ID NO: 44);

βDR-3,4,5=AGTTCANnTGAACT, wherein n is 3, 4 or 5 (SEQ ID NO: 22); and

IR-6=TGAACTNnAGGTCA, wherein n is 6 (SEQ ID NO: 23), and the like.

Those of skill in the art recognize that any combination of nucleotidescan be used to make up the 3, 4, 5, or 6 nucleotide spacer between therepeated half sites (e.g., N_(n) in SEQ ID NOS: 15, 16, 17, 22 or 23).

Such response elements are generally found in genes encoding catabolicenzymes, such as CYP2A1, CYP2A2, CYP2C1, CYP3A1, CYP3A2, an P450oxidoreductase, uridine diphosphate glucuronosyltransferase, aglucuronosyl transferase, and the like, transcription of which genes isactivated or suppressed by practice of the invention method(s).

Representative examples of agonists capable of activating transcriptionof such catabolic enzymes include molecules that have high-affinityreceptors, such as progesterone, testosterone, estrogen andcorticosterone, as well as their reduced catabolites that are, for themost part, inactive on the high-affinity receptors. In addition to thenatural steroids, SXR is activated by synthetic steroids, including PCNand dexamethasone, as well as by xenobiotic drugs, phytosteroids, andthe like. The presently preferred agonists include corticosterone,rifampicin, nifedipine, corticosterone, DES, estradiol,dihydrotestosterone, pregnenolone, progesterone, and PCN, withcorticosterone being the strongest known activator.

When the modulator is an antagonist of SXR, the modulator functions inone or more of the following ways: (1) to block binding of thepolypeptide to the SXR response element, (2) to inhibit formation of aheterodimer of the polypeptide and a retinoid X receptor, or (3) toinhibit binding of a ligand to the ligand binding domain of SXR or aninvention SXR polypeptide. For example, the antagonist can inhibitformation of a heterodimer between a retinoid X receptor and SXR or aninvention SXR polypeptide by blocking the docking site between themolecules. Alternatively, an antagonist can inhibit binding of a ligandto the ligand binding domain of SXR or invention SXR polypeptide bybinding to the active site of the ligand (i.e., the portion of theligand that binds to the ligand binding domain). Any of a variety ofcompounds that will accomplish one or more of these goals can be used asan antagonist in the invention methods. For example, an antibody thatbinds to SXR or to a RXR so as to prevent formation of an SXR:RXRheterodimer can be used as an antagonist in the practice of the presentinvention. Similarly, an antibody that blocks the ligand binding domainof SXR without activating transcription of the target gene so as toprevent binding of the ligand to the ligand binding domain will functionas an antagonist in the invention method(s).

One of skill in the art will be aware of, or can readily devise,additional polypeptides or nucleotides that will act as antagonists ofgene transcription in the invention method(s).

In accordance with another embodiment of the present invention, thereare provided heterodimer complexes which consist of the above-describedreceptor polypeptide and RXR or other silent partner therefor.

In accordance with yet another embodiment of the present invention,there are provided isolated nucleic acids which encode theabove-described receptor polypeptides. As used herein, the phrase“isolated nucleic acid” means a nucleic acid that is in a form that doesnot occur in nature. One means of isolating a nucleic acid encoding apolypeptide is to probe a mammalian genomic library with a natural orartificially designed DNA probe using methods well known in the art. DNAprobes derived from the SXR gene are particularly useful for thispurpose. DNA and cDNA molecules that encode SXR polypeptides can be usedto obtain complementary genomic DNA, cDNA or RNA from human, mammalian(e.g., mouse, rat, rabbit, pig, and the like), or other animal sources,or to isolate related cDNA or genomic clones by the screening of cDNA orgenomic libraries, by methods described in more detail below. Examplesof nucleic acids are RNA, cDNA, or isolated genomic DNA encoding SXR.

Exemplary DNAs include those which encode substantially the same aminoacid sequence as shown in SEQ ID NO:2 (e.g., a contiguous nucleotidesequence which is substantially the same as nucleotides 583-1884 shownin SEQ ID NO:1). Presently preferred DNAs include those which encode thesame amino acid sequence as shown in SEQ ID NO:2 (e.g., a contiguousnucleotide sequence which is the same as nucleotides 583-1884 shown inSEQ ID NO:1).

As used herein, nucleotide sequences which are substantially the sameshare at least about 90% identity, and amino acid sequences which aresubstantially the same typically share more than 95% amino acididentity. It is recognized, however, that proteins (and DNA or mRNAencoding such proteins) containing less than the above-described levelof homology arising as splice variants or that are modified byconservative amino acid substitutions (or substitution of degeneratecodons) are contemplated to be within the scope of the presentinvention. As readily recognized by those of skill in the art, variousways have been devised to align sequences for comparison, e.g., theBlosum 62 scoring matrix, as described by Henikoff and Henikoff in Proc.Natl. Acad. Sci. USA 89:10915 (1992). Algorithms conveniently employedfor this purpose are widely available (see, for example, Needleman andWunsch in J. Mol. Biol. 48:443 (1970).

In accordance with still another embodiment of the present invention,there are provided nucleic acid constructs comprising theabove-described nucleic acid, operatively linked to regulatoryelement(s) operative for transcription of the nucleic acid andexpression of the polypeptide in an animal cell in culture. There arealso provided cells containing such a construct, optionally containing areporter vector comprising:

-   -   (a) a promoter that is operable in said cell,    -   (b) a SXR response element, and    -   (c) DNA encoding a reporter protein,    -   wherein the reporter protein-encoding DNA is operatively linked        to the promoter for transcription of the DNA, and    -   wherein the promoter is operatively linked to the SXR response        element for activation thereof.

In accordance with a further embodiment of the present invention, thereare provided methods of making invention receptor polypeptide(s), saidmethods comprising culturing cells containing an expression vectoroperable in said cells to express a DNA sequence encoding saidpolypeptide.

In accordance with a still further embodiment of the present invention,there are provided probes comprising labeled single-stranded nucleicacid, comprising at least 20 contiguous bases in length havingsubstantially the same sequence as any 20 or more contiguous basesselected from bases 1-2068, inclusive, of the DNA illustrated in SEQ IDNO:1, or the complement thereof. An especially preferred probe of theinvention comprises at least 20 contiguous bases in length havingsubstantially the same sequence as any 20 or more contiguous basesselected from bases 583-1884, inclusive, of the DNA illustrated in SEQID NO:1, or the complement thereof.

Those of skill in the art recognize that probes as described herein canbe labeled with a variety of labels, such as for example, radioactivelabels, enzymatically active labels, fluorescent labels, and the like. Apresently preferred means to label such probes is with ³²P. Such probesare useful, for example, for the identification of receptorpolypeptide(s) characterized by being responsive to the presence of oneor more steroid and/or xenobiotic to regulate the transcription ofassociated gene(s), said method comprising hybridizing test DNA with aprobe as described herein under high stringency conditions (e.g.,contacting probe and test DNA at 65° C. in 0.5 M NaPO₄, pH 7.3, 7%sodium dodecyl sulfate (SDS) and 5% dextran sulfate for 12-24 hours;washing is then carried out at 60° C. in 0.1×SSC, 0.1% SDS for threethirty minute periods, utilizing fresh buffer at the beginning of eachwash), and thereafter selecting those sequences which hybridize to saidprobe.

In another aspect of the invention, the above-described probes can beused to identify invention receptor polypeptide(s), or functionalfragments thereof, said methods comprising hybridizing test DNA with aprobe as described herein under high stringency conditions, andselecting those sequences which hybridize to said probe.

In yet another aspect of the invention, the above-described probes canbe used to assess the tissue sensitivity of an individual to exposure tosteroid and steroid-like compounds by determining SXR mRNA levels in agiven tissue sample. It is expected that an individual having a highlevel of SXR mRNA (or protein) will be sensitive to the presence ofsignificant levels of steroid and xenobiotic compounds, such as areencountered in many foods, or as a result of overproduction and/orreduced ability to degrade steroids, as seen in such diseases asCushing's syndrome, virilism and hirsutism in females, polycysticovarian syndrome, and the like.

In accordance with yet another embodiment of the present invention,there are provided antibodies which specifically bind theabove-described receptor polypeptides. Preferably, such antibodies willbe monoclonal antibodies. Those of skill in the art can readily preparesuch antibodies having access to the sequence information providedherein regarding invention receptors.

Thus, the above-described antibodies can be prepared employing standardtechniques, as are well known to those of skill in the art, using theinvention receptor proteins or portions thereof as antigens for antibodyproduction. Both anti-peptide and anti-fusion protein antibodies can beused (see, for example, Bahouth et al. Trends Pharmacol Sci. 12:338-343(1991); Current Protocols in Molecular Biology (Ausubel et al., eds.)John Wiley and Sons, New York (1989)). Factors to consider in selectingportions of the invention receptors for use as immunogen (as either asynthetic peptide or a recombinantly produced bacterial fusion protein)include antigenicity, uniqueness to the particular subtype, and thelike.

The availability of such antibodies makes possible the application ofthe technique of immunohistochemistry to monitor the distribution andexpression density of invention receptors. Such antibodies could also beemployed for a variety of other uses, e.g., for diagnostic andtherapeutic applications.

In accordance with a further embodiment of the present invention,binding assays employing SXRs are provided, useful for rapidly screeninga large number of compounds to determine which compounds (e.g., agonistsand antagonists) are capable of binding to the receptors of theinvention. Subsequently, more detailed assays can be carried out withinitially identified compounds, to further determine whether suchcompounds act as agonists or antagonists of invention receptors.

The invention binding assays may also be employed to identify newSXR-like ligands. Test samples (e.g., biological fluids) may also besubjected to invention binding assays to detect the presence or absenceof SXR or SXR ligands.

Another application of the binding assay of the invention is the assayof test samples (e.g., biological fluids) for the presence or absence ofSXR. Thus, for example, tissue homogenates from a patient displayingsymptoms thought to be related to over- or under-production of steroidscan be assayed to determine if the observed symptoms are related to thepresence of SXR.

The binding assays contemplated by the present invention can be carriedout in a variety of ways, as can readily be identified by one of skillin the art. For example, competitive binding assays can be employed, aswell as radioimmunoassays, ELISA, ERMA, and the like.

In accordance with yet another embodiment of the present invention,there is provided a method of testing a compound for its ability toregulate transcription-activating effects of invention receptorpolypeptide(s), said method comprising assaying for the presence orabsence of reporter protein upon contacting of cells containing saidreceptor polypeptide and reporter vector with said compound;

-   -   wherein said reporter vector comprises:    -   (a) a promoter that is operable in said cell,    -   (b) a hormone response element, and    -   (c) DNA encoding a reporter protein,    -   wherein said reporter protein-encoding DNA is operatively linked        to said promoter for transcription of said DNA, and    -   wherein said promoter is operatively linked to said hormone        response element for activation thereof.

Hormone response elements suitable for use in the above-described assaymethod comprise direct or inverted repeats of at least two half sites(each having the sequence RGBNNM, as defined herein). In each half site,RGBNNM:

-   -   R is selected from A or G;    -   B is selected from G, C, or T;    -   each N is independently selected from A, T, C, or G; and    -   M is selected from A or C;    -   with the proviso that at least 4 nucleotides of said -RGBNNM-        sequence are identical with the nucleotides at corresponding        positions of the sequence AGTTCA.

Optionally, the above-described method of testing can be carried out inthe further presence of ligand for invention receptors, thereby allowingthe identification of antagonists of invention receptors. Those of skillin the art can readily carry out antagonist screens using methods wellknown in the art. Typically, antagonist screens are carried out using aconstant amount of agonist, and increasing amounts of a putativeantagonist (i.e., a competitive assay). Alternatively, antagonists canbe identified by rendering the receptor constitutively active (e.g., byadding a strong, constitutively-active activator to the receptor) andscreening for compounds which shut down the resultingconstitutively-active receptor.

In accordance with another aspect of the present invention, there areprovided methods to identify compounds which are agonists of steroid Xreceptor (SXR), but which neither agonize nor antagonize other steroidreceptors, said method comprising:

-   -   detecting in a first assay system the presence or absence of        reporter protein upon contacting of cells containing SXR and        reporter vector with said compound;    -   wherein said reporter vector comprises:    -   (a) a promoter that is operable in said cell,    -   (b) an SXR response element, and    -   (c) DNA encoding a reporter protein,    -   wherein said reporter protein-encoding DNA is operatively linked        to said promoter for transcription of said DNA, and    -   wherein said promoter is operatively linked to said SXR response        element for activation thereof;

detecting in a second assay system the presence or absence of reporterprotein upon contacting of cells containing a steroid hormone receptorother than SXR and reporter vector with said compound;

-   -   wherein said reporter vector comprises:    -   (a) a promoter that is operable in said cell,    -   (b) a response element for said receptor other than SXR, and    -   (c) DNA encoding a reporter protein,    -   wherein said reporter protein-encoding DNA is operatively linked        to said promoter for transcription of said DNA, and    -   wherein said promoter is operatively linked to said response        element for said receptor other than SXR for activation thereof;        and

identifying those compounds which induce production of reporter in saidfirst assay, but not in said second assay, as compounds which areagonists of steroid X receptor (SXR), but neither agonists norantagonists of other steroid receptors.

Thus, it can readily be seen that invention methods can be used toidentify a variety of therapeutically useful compounds. The compoundsidentified as described herein can be used for the treatment of a widevariety of indications, such as, for example:

-   -   a) Cushing's syndrome (hypercortisolism), which manifests as        increased cortisol levels, leading to numerous problems        including obesity, fatigue, hypertension, edema and        osteoporosis;    -   b) virilism and hirsutism in females due to overproduction of        testosterone;    -   c) androgen excess due to polycystic ovarian syndrome, which        manifests as greatly increased circulating levels of        dehydroepiandrosterone;    -   d) enzymatic defects which lead to accumulation of specific        steroids, such as:        -   1) 21-hydroxylase deficiency leading to increased synthesis            of 17-hydroxy-progesterone and androgens;        -   2) 11β-hydroxylase deficiency leading to deoxycortisol and            deoxycorticosterone accumulation and attendant hypertension;        -   3) 3β-hydroxysteroid dehydrogenase deficiency resulting in            accumulation of pregnenolone and dehydroepi-androsterone,            leading to sexual ambiguity in both sexes;        -   4) 17-hydroxylase deficiency, which prevents cortisol            synthesis but leads to accumulation of corticosterone and            deoxycorticosterone, resulting in hypertension and aberrant            development of secondary sexual characteristics in both            sexes;    -   f) ameliorate the effect of substances in the diet and/or        environment which act as endocrine disruptors, e.g., estrogens        which may be involved in breast, colorectal and prostate cancers        (Adlercreutz and Mazur in Ann. Med. 29:95-120 (1997); and the        like.

Compounds which are specific agonists for SXR without acting as eitheragonists or antagonists for other steroid receptors will find particularutility where other steroid compounds have been used for their catatoxicproperties, while tolerating the negative effects of such therapeuticuse (presumably caused by the undesirable activation of previouslydescribed steroid receptors, e.g., glucocorticoid receptor).

In accordance with a still further embodiment of the present invention,there are provided methods for modulating process(es) mediated byinvention receptor polypeptides, said methods comprising conducting saidprocess(es) in the presence of at least one agonist, antagonist orantibody raised against invention receptor.

In accordance with yet another embodiment of the present invention,there are provided methods for inducing the expression of steroiddegradative enzymes, said method comprising activating SXR. Exemplarysteroid degradative enzymes contemplated for expression herein includesteroid hydroxylases, and the like.

In accordance with the present invention, it has further been discoveredthat induction of some xenobiotic-metabolizing enzymes bypharmacological levels of steroids is regulated by SXR, a class ofbroad-specificity, low-affinity, nuclear hormone receptors. One benefitof such a receptor-based system is that it induces the expression ofxenobiotic metabolizing enzymes only at activator levels sufficientlyhigh to interfere with normal endocrine function. It also makesbiological sense that the expression of enzymes with broad substratespecificity, such as cytochrome P450s, can be induced by a receptorresponsive to a diverse group of activators, some of which can besubstrates for the induced enzymes.

To determine whether the activity of SXR was ligand-dependent, mixturesof natural and synthetic compounds were tested for their ability toactivate SXR in transfection-based assays (see Example 3). A mixturecontaining dehydroepiandrosterone (DHEA) and pregnenolone was observedto be active, suggesting that SXR might be a new steroid receptor. Tocharacterize its response properties, a large variety of steroids,including intermediate and major products of known steroid biosyntheticpathways, were tested. Surprisingly, most of these compounds wereactive, although there were clear differences in potency (see FIG. 2).Indeed, most of the more than 70 steroids tested showed some activity athigh doses. Activation was dependent on the ligand binding domain of SXRsince both full-length receptors and GAL4-receptor ligand binding domainchimeras showed similar activity, whereas there was no activation ofreporter gene expression in experiments with reporter alone or reporterplus GAL4 DNA-binding domain.

The most potent and efficacious activator of the numerous steroidstested is corticosterone. Estradiol and dihydrotestosterone are alsoremarkably effective activators while aldosterone and 1,25 dihydroxyvitamin D3 are inactive, even at 50 mM. Although ligands for theclassical steroid receptors do show some overlap in receptorspecificity, there is no example of a nuclear receptor that can beactivated by so many different types of steroids. This broad ligandspecificity of SXR parallels that of PPARα, which can be activated by anextremely diverse group of dietary fatty acids at micromolar levels(see, for example, Forman et al., in Proc. Natl. Acad. Sci. USA 94:4312(1997) and Gottlicher et al., in Proc. Natl. Acad. Sci. USA 89:4653(1992)).

The diversity of steroids showing activity on SXR suggests that thisnovel class of receptors might be able to sense cumulative, as well asindividual steroid levels, predicting that combinations of activatorsmight be more active than the individual components. As shown in FIG. 3,a cocktail containing 10 steroids, each at 10 mM concentration (i.e., anoverall steroid concentration of 100 mM), was considerably more activethan its individual components at 10 mM, a concentration at which mostwere inactive. These results confirm that SXR is a broad-specificity,low-affinity, steroid-activated receptor.

An important requirement for physiologic homeostasis is the removal anddetoxification of various endogenous hormones and xenobiotic compoundswith biological activity. Much of the detoxification is performed bycytochrome P450 enzymes, many of which have broad substrate specificityand are inducible by a bewildering array of compounds, includingsteroids. The ingestion of dietary steroids and lipids induces the sameenzymes and thus, must be integrated into a coordinated metabolicpathway. Instead of possessing hundreds of receptors, one for eachinducing compound, the class of receptors described herein indicates theexistence of a class of broad-specificity, low-affinity nuclearreceptors that monitor total steroid levels and induce the expression ofgenes encoding xenobiotic metabolizing enzymes. These results indicatethe existence of a steroid sensor mechanism for removal of elevatedlevels of steroids (or steroid-like compounds) from circulation viabroad-specificity, low-affinity receptors which represent a novel branchof the nuclear receptor superfamily.

A search of the GENBANK database for genes containing potential SXRresponse elements identified the steroid hydroxylases CYP2A1, CYP2A2,CYP2C1, CYP2C6, CYP3A1, CYP3A2, P450 oxidoreductase, andUDP-glucuronosyl-transferase as candidate target genes (FIG. 6A). Thesearch identified DR-3, DR-4 and DR-5 elements present in these genes,which indicates that such compounds activate the invention SXR.Similarly, the transfection-based assays described in Example 4, whichwere conducted to test the ability of steroids and xenobiotics toactivate SXR response elements showed that corticosterone along withpregnenolone, progesterone, dihydrotestosterone (DHT), estradiol, andPCN are consistently among the best activators. Dexamethasone,cortisone, and DHEA are in the group of intermediate activators, andthere is little response from either aldosterone or cortisol (FIG. 4).Consistent with the DNA-binding data, maximal activities induced bythese activators was achieved in steroid inducible P450 genes containingβDR-3, βDR-4, and βDR-5 response elements (FIG. 4).

Indeed, a search of the GENBANK database for genes containing putativeSXR response elements identified a number of steroid hydroxylases, e.g.,CYP2A1, CYP2A2, CYP2C1, CYP2C6, CYP3A1, CYP3A2, P450 oxidoreductase andUDP-glucuronosyltransferase, as candidate target genes. The relevantportions of these sequences are as follows:

DR-3 rCYP3A1 (SEQ ID NO:3) tagac AGTTCA tga AGTTCA tctac rCYP3A2 (SEQ IDNO:4) taagc AGTTCA taa AGTTCA tctac rUGT1A6 (SEQ ID NO:5) actgt AGTTCAtaa AGTTCA catgg DR-4 rbCYP2C1 (SEQ ID NO:6) caatc AGTTCA acag GGTTCAccaat rP450R (SEQ ID NO:7) cac AGGTGA gctg AGGCCA gcagc AGGTCG aaa DR-5rCYP2A1 (SEQ ID NO:8) gtgca GGTTCA actgg AGGTCA acatg rCYP2A2 (SEQ IDNO:9) gtgct GGTTCA actgg AGGTCA gtatg rCYP2C6 (SEQ ID NO:10) agtctAGTTCA gtggg GGTTCA gtctt hCYP2E1 (SEQ ID NO:11) gagat GGTTCA aggaaGGGTCA ttaac

The data shown in FIG. 4 verify that SXR can activate DR-3, DR-4 andDR-5 elements that are present in these genes. In the series oftransfections described in Example 3, corticosterone, along withpregnenolone, progesterone, DHT, estradiol and PCN, are consistentlyamong the best activators. Dexamethasone, cortisone and DHEA are in theintermediate group, with little response from either aldosterone orcortisol (see FIG. 4). Consistent with the DNA-binding data, maximalactivities are achieved on DR-3, DR-4 and DR-5 elements.

Thus, SXR response elements are found in genes encoding steroidhydroxylases, P450 oxidoreductase, and glucuronosyl transferase. Theseenzymes can metabolize endogenous as well as xenobiotic compounds andare legitimate targets for a receptor that is activated bypharmacological levels of steroids. SXR is highly expressed in liver,the major expression site of xenobiotic metabolizing enzymes, suggestingthat the steroid sensor mechanism is active in the appropriate tissue.In addition, prominent expression is also found in the intestine.Although less is known about the role of this tissue in steroid orxenobiotic metabolism, it is certainly possible that the intestine playsa role in regulating the metabolism of dietary, and perhaps endogenous,steroids. Taken together, these data strongly support the existence of aclass of low-affinity, broad-specificity nuclear hormone receptor(s),such as SXR, which function as intracellular “steroid sensor(s)”.

The localization of apparent SXR-responsive elements in genes encodingsteroid hydroxylases raises the question of whether products of steroidcatabolism, such as reduced or hydroxylated corticosterone derivatives,could also activate SXR. FIG. 5 shows that both 5α and 5β reduced formsof corticosterone are effective SXR activators whereas 5α is slightlyactive and 5β is completely inactive on GR. While a few 5α-reducedsteroids remain active (e.g., dihydrotestosterone), virtually all5β-reduced steroids are unable to activate classical steroid receptors(see Russell and Wilson in Ann. Rev. Biochem. 63:25 (1994)).Accordingly, the activation of SXR by 5β-reduced steroids reveals apreviously unidentified role for these compounds in gene regulation.

6β-hydroxy corticosterone is virtually inactive on SXR and slightlyactive on GR (see FIG. 5). CYP3A genes, which contain SXR-activatableresponse elements, catalyze the hydroxylation of many steroids at the 6position. Therefore, the inability of 6β-hydroxy-corticosterone toactivate SXR suggests that 6-hydroxylation is a potential regulatorystep in the SXR signaling pathway.

Thus, in support of the role for members of the SXR class of nuclearreceptors proposed herein, it has been demonstrated herein that SXR isactivated by an extremely diverse group of steroids and theirmetabolites, including molecules that have high-affinity receptors suchas progesterone, testosterone, estrogen and corticosterone as well astheir reduced catabolites that are, for the most part, inactive on thehigh-affinity receptors. In addition to the natural steroids, SXR isactivated by synthetic steroids including PCN and dexamethasone. Thesedata provide a molecular explanation for the paradoxical induction ofthe CYP3A genes (a.k.a. P450_(PCN)) by both glucocorticoid receptoragonists and antagonists since the cyp3A genes harbor a SXR-activatableresponse element in the promoter region that has been shown to beresponsible for PCN and glucocorticoid induction (see Burger et al.supra and Gonzalez et al. supra). Whereas such a result is unexplainableby regulation of traditional, high-affinity steroid receptors, suchbehavior is consistent with the observed properties of the newlycharacterized steroid X receptor.

Further tests were conducted to discover whether P450s known to beinducible by PCN and other steroids could be SXR targets. The primaryhuman steroid-inducible P450 is the CYP3A4 gene (Molowa et al., Proc.Natl. Acad. Sci. (USA) 83:5311-5315, 1986, Beaune et al., Proc. Natl.Acad. Sci. (USA) 83:8064-8068, 1986). Unlike the rat and mouse CYP3Agenes, all of which contain a DR-3 response element that SXR canactivate (FIG. 4), the human and rabbit promoters do not contain such anelement. Inducibility of CYP3A4 by steroids and xenobiotics has beenlocalized to a 19 base pair element that is functional in transienttransfection assays (Barwick et al., Mol. Pharmacol. 50:10-16, 1996).This element contains the IR-6 motif (TGAACTcaaaggAGGTCA) (SEQ IDNO:24). Similar elements have been identified in human CYP3A5, andCYP3A7 and in rabbit CYP3A6 genes (FIG. 6B) (Barwick, supra, 1996).Tests conducted to determine the ability of SXR to bind a series ofinverted repeat elements with spacings from zero to six nucleotidesdetermined that only an IR-6 response element, showed significantbinding. As with the direct repeats, these results indicate the bindingwas dependent on formation of a RXR:SXR heterodimer. In addition,competition binding experiments demonstrated little difference in theapparent affinity of SXR:RXR heterodimers for the DR-4 and CYP3A4 IR-6response elements. In accord with the known inducibility of the parentpromoters, SXR was shown to activate reporter constructs containing theCYP3A4, but not the CYP3A5 or CYP3A7 motifs.

Compounds known to induce CYP3A4 were also shown to activate theinvention SXR. The compounds tested included drugs, such as rifampicinand nifedipine; steroid antagonists, such as tamoxifen, spironolactoneand PCN; natural and synthetic steroids, such as dexamethasone,diethylstilbestrol, estradiol, dihydrotestosterone, corticosterone andcortisone; and phytoestrogens, such as coumestrol, equol and genistein.Of these compounds, rifampicin, nifedipine, corticosterone, estradiol,DES, and coumestrol were the most potent activators (FIG. 7A). The mousereceptor PXR responded poorly to these inducers, but was preferentiallyactivated by PCN, a weak activator of SXR (FIG. 7B). PXR is reported tobe preferentially activated by pregnanes (21-carbon steroids such asdexamethasone (DEX) and pregnenolone) (Kliewer, supra, 1998); however,the tests described herein showed that PXR is similarly activated by19-carbon androstanes, like testosterone, and 18-carbon estranes, likeestradiol (FIG. 7B). Similar results were obtained with other naturalsteroids, including progesterone, pregnenolone and dihydroethanoic acid(DHEA).

To demonstrate that the activation of SXR and PXR by high steroidconcentrations is not a general property of all steroid receptors,parallel tests were conducted to determine the activation of the humanestrogen receptor (ER) by the same panel of compounds. The onlyendogenous steroids tested that activated the ER were DHT and estradiol.The synthetic ER agonist, DES, and the phytoestrogens, includingcoumestrol (FIG. 7C), also activated the human estrogen receptor.

Because the invention SXR-responsive elements are localized in genesencoding steroid hydroxylases, products of steroid catabolism, such asreduced or hydroxylated corticosterone derivatives, were tested foractivation of SXR. The results of these tests shown in FIG. 7Dillustrate that both 5α and 5β reduced forms of corticosterone areeffective SXR activators; however, 5α is slightly active, and 5β iscompletely inactive on GR. While a few 5α-reduced steroids remain active(e.g., dihydrotestosterone), 5β-reduced steroids fail to activateclassical steroid receptors (Russell and Wilson, Ann. Rev. Biochem.63:25-61, 1994). Therefore, the activation of SXR by 5β-reduced steroidsmay reflect a previously undetected regulatory pathway for thesecompounds. In addition, the virtual inactivity of 6β-hydroxycorticosterone on SXR (FIG. 6D) suggests that CYP3A4 catalyzedhydroxylation is a potential definitive regulatory step in steroidmetabolism.

These results indicate that the induction of somexenobiotic-metabolizing enzymes by pharmacological levels of steroids,drugs, and xenobiotic compounds is regulated by a broad-specificitysensor, rather than numerous specific receptors. SXR is a novel memberof the nuclear receptor superfamily that is activated by a diverse groupof steroids and their metabolites. Direct regulation by abroad-specificity sensor, such as the invention SXR, is biologicallyeconomical since much of the detoxification and catabolism of suchcompounds is mediated by cytochrome P450 enzymes, particularly membersof the CYP3A family, which both metabolize, and are induced by, a widespectrum of diverse compounds, including steroids.

Based on the above-described studies, a number of relationships havebeen discovered among target genes, the SXR, and its activators thatsupport the role of the SXR as a broad sensitivity sensor responsiblefor regulating cumulative levels of steroids and xenobiotics. First, SXRis expressed in tissues which catabolize steroids and xenobiotics,particularly in liver, the major expression site of steroid andxenobiotic metabolizing enzymes, and in the intestine. Although less isknown about the role of gut tissue in steroid metabolism, the gut isknown to play an important role in first pass metabolism of dietary, andorally-administered compounds (Holtbecker et al., Drug Metab. Dispos.24:1121-1123, 1996; and Kolars et al., Lancet 338:1488-1490, 1991). Forexample, CYP3A4 is highly expressed in enterocytes (Kolars et al., J.Clin. Invest. 90:1871-1878, 1992). Thus, SXR is expressed at high levelsin two key tissues for steroid and xenobiotic catabolism. Second,catabolic enzymes expressed in tissues that express SXR are induced bythe invention SXR. SXR response elements have been discovered in thewell-characterized CYP3A4 promoter as well as those of P450oxidoreductase, CYP2A, CYP2C, CYP2E and glucuronosyl transferase, whichare all known to be involved in steroid and xenobiotic catabolism (F. J.Gonzalez, Trends Pharmacol. Sci. 13:346-352, 1992). Third, compoundsknown to induce catabolic enzymes activate the invention SXR, includingdrugs (such as rifampicin and nifedipine), steroid receptor agonists andantagonists (such as estrogen and tamoxifen); bioactive dietarycompounds (such as phytoestrogens), and the like. In particular, CYP3A4is known to be inducible (Rendic and Di Carlo, 1997) by virtually allthe compounds applicants have identified as SXR activators. Lastly,products of early catabolic steps, such as reduced steroids, activateSXR, ensuring their complete inactivation and elimination. Takentogether, these relationships support the role of the SXR as abroad-specificity sensor operative to regulate homeostasis of steroidsand xenobiotics.

Activation of SXR also provides a molecular explanation for theparadoxical induction of the CYP3A genes (a.k.a. P450_(PCN)) by bothglucocorticoid receptor agonists and antagonists and for thedifferential response of orthologous enzymes in different species. Theinducible CYP3A genes harbor a response element in their promoters thathas been shown to be responsible for PCN and glucocorticoid induction(Barwick, supra, 1996; Burger, supra, 1992; Gonzalez, supra, 1986;Schuetz and Guzelian, supra, 1984; and Kliewer, supra, 1998). Applicantshave discovered that these response elements can be activated by theinvention SXR (FIGS. 6A and 6C). Despite their common role in steroidand xenobiotic catabolism, CYP3A genes from different species, andparticularly the glucocorticoid-responsive promoter elements, showconsiderable differences in the pharmacology of their inducers (Barwick,supra, 1996). For example, PCN is a strong inducer of rat CYP3A2 andCYP3A3, but a weak inducer of human CYP3A4 and rabbit CYP3A6. On theother hand, rifampicin is a strong inducer of the human and rabbit genesencoding such enzymes but not the rat genes (Barwick, supra, 1996).

However, when the response elements from such genes are tested bytransient transfection into primary hepatocytes from rats or rabbits,the responsiveness changes to that of the host cell type. For example,glucocorticoid-responsive elements from the rat CYP3A2 and CYP3A3promoters were induced by DEX in both rat and rabbit hepatocytes, by PCNonly in rat hepatocytes, and by rifampicin only in rabbit hepatocytes(Barwick, supra, 1996). Similarly, the glucocorticoid-responsive elementfrom the human CYP3A4 promoter was inducible by DEX in both rat andrabbit hepatocytes, by PCN only in rat hepatocytes, and by rifampicinonly in rabbit hepatocytes (Barwick, supra, 1996). The activationprofiles in rat cells correspond to the responsiveness of PXR to theinducers (FIG. 6C); whereas the responsiveness in rabbit cellscorresponds to that of SXR. Since the rabbit 3A6 promoter lacks therodent DR-3 element, but has the human IR-6 element (Barwick, supra,1996), it can be inferred that rabbit liver will likely have a receptormore closely related to SXR than to PXR. Thus, the pharmacology of SXRand PXR activation explains the different inducibility of the rat,rabbit, and human members of the cytochrome P4503A family. Thisdiscovery suggests that rabbit hepatocytes behave more like their humancounterparts than do rodent hepatocytes, and that rabbits are perhapsbetter suited to testing for human-like drug interaction than rodents.

One additional member of the new branch of the nuclear receptorsuperfamily called the steroid and xenobiotic receptor has beendiscovered in mouse tissue. Screening of a mouse liver cDNA library atreduced stringency resulted in the identification of 39 cDNAs, all ofwhich encoded PXR.1. Orthologous nuclear receptors typically sharegreater than 90% amino acid identity in the ligand binding domain whencomparing rodent and human receptors (e.g., RARα-98% human/mouse (h/m),PPARγ-98% h/m, GR-95% h/m, TRβ-98% h/m, ERα-89% h/m). Therefore, PXR andSXR may represent α and β subtypes of the steroid and xenobiotic nuclearreceptor family. This conclusion is supported by the distinctpharmacological properties of the receptors, as illustrated in theExamples herein. Further screening of mouse and human liver cDNAlibraries has failed to identify other family members. It is alsopossible that PXR and SXR represent unusually divergent orthologousgenes. If this were correct, the divergence might reflect adaptation ofthe receptor to the difference between the diets of rodents and primatesand the requirement for the receptor to respond to appropriatefood-borne compounds.

To obtain the invention receptor, commercially obtained Northern blotsof multiple human tissues were probed by full-length SXR cDNA (SEQ IDNO: 1), as described in Example 1 herein. The results showed that SXRmRNA is expressed at high levels in human liver and at more moderatelevels in human intestine. Exposures of the Northern blots for longerthan 24 hours did not reveal expression in any other tissues. MultiplemRNAs were detected, ranging from 3500 nt to larger than 9000 nt.Comparison of the sequences of the four cDNAs obtained reveals sharedprotein coding and 5′ untranslated sequences, but a different 3′ end foreach of the four. These sequence differences may be due to alternativepolyadenylation.

Electrophoretic mobility shift assays were employed to determine theability of SXR to heterodimerize with RXR and to analyze the selectivityand specificity of SXR DNA binding as described in Example 4 herein.Receptors that heterodimerize with RXR typically bind to direct repeatsof AGGTCA or closely related sequences (Mangelsdorf and Evans, supra,1995). SXR alone and in combination with RXR was tested against a seriesof response elements differing in the spacing between half sites from 0to 15 nucleotides. No binding was seen on classic steroid responseelements. In contrast, strong binding was selective to a DR-4 motif withminimal binding to DR-3 and DR-5, and no binding to other spacings. Whenthe variant AGTTCA half site was used, strong binding was seen on DR-4and DR-5, and significant, but reduced, binding to DR-3. These resultsdemonstrate that SXR binds DNA as a heterodimer with RXR rather than asa homodimer like the classical steroid receptors (Beato, supra, 1995).

The term “effective amount” as applied to a SXR polypeptide agonist orantagonist according to the invention means the quantity necessary tomodulate metabolism of one or more steroid and/or xenobiotic compoundsto a desired level, for example, a level effective to treat, cure, oralleviate the symptoms of a disease state for which the therapeuticcompound is being administered, or to establish homeostasis.Alternatively, when an agonist according to the invention is employed toprevent steroid toxicity in a subject therapeutically administered oneor more therapeutic steroid and/or xenobiotic compounds in treatment ofa disease state, the term “effective amount” is an amount necessary tobring the overall level of steroids and xenobiotic compounds to a safelevel, for example as determined by blood tests of the individual beingtreated for the effects of steroid toxicity, or to alleviate thesymptoms of steroid toxicity as determined by the physician. Similarly,the amount of a SXR polypeptide antagonist according to the inventionused to slow clearance of a therapeutic steroid or xenobiotic compoundis an amount necessary to raise the blood level of the particulartherapeutic compound to a therapeutic level and hence treat or alleviatethe symptoms of the disease state for which the therapeutic steroid orxenobiotic compound is being administered. Since individual subjects maypresent a wide variation in severity of symptoms and each drug or activeagent has its unique therapeutic characteristics, the precise mode ofadministration, dosage employed and treatment protocol for each subjectis left to the discretion of the practitioner.

Amounts effective for the particular therapeutic goal sought will, ofcourse, depend on the severity of the condition being treated, and theweight and general state of the subject. Various general considerationstaken into account in determining the “effective amount” are known tothose of skill in the art and are described, e.g., in Gilman et al.,eds., Goodman And Gilman's: The Pharmacological Bases of Therapeutics,8th ed., Pergamon Press, 1990; and Remington's Pharmaceutical Sciences,17th ed., Mack Publishing Co., Easton, Pa., 1990, each of which isherein incorporated by reference.

Pharmaceutical formulations of the SXR polypeptide agonists orantagonists of the present invention can be used in the form of a solid,a solution, an emulsion, a dispersion, a micelle, a liposome, and thelike, wherein the resulting formulation contains one or more of theagonists or antagonists contemplated for use in the practice of thepresent invention, as active ingredients, in admixture with an organicor inorganic carrier or excipient suitable for enteral or parenteralapplications. The active ingredients may be compounded, for example,with the usual non-toxic, pharmaceutically acceptable carriers fortablets, pellets, capsules, suppositories, solutions, emulsions,suspensions, and any other form suitable for use. The carriers which canbe used include glucose, lactose, gum acacia, gelatin, mannitol, starchpaste, magnesium trisilicate, talc, corn starch, keratin, colloidalsilica, potato starch, urea, medium chain length triglycerides,dextrans, and other carriers suitable for use in manufacturingpreparations, in solid, semisolid, or liquid form. In additionauxiliary, stabilizing, thickening and coloring agents and perfumes maybe used. The active compounds (i.e., one or more SXR polypeptide agonistor antagonist) are included in the pharmaceutical formulation in anamount sufficient to produce the desired effect upon the target process,condition or disease.

Pharmaceutical formulations containing the active ingredientscontemplated herein may be in a form suitable for oral use, for example,as tablets, troches, lozenges, aqueous or oily suspensions, dispersiblepowders or granules, emulsions, hard or soft capsules, or syrups orelixirs. Formulations intended for oral use may be prepared according toany method known in the art for the manufacture of pharmaceuticalformulations. In addition, such formulations may contain one or moreagents selected from a sweetening agent (such as sucrose, lactose, orsaccharin), flavoring agents (such as peppermint, oil of wintergreen orcherry), coloring agents and preserving agents, and the like, in orderto provide pharmaceutically elegant and palatable preparations. Tabletscontaining the active ingredients in admixture with non-toxicpharmaceutically acceptable excipients may also be manufactured by knownmethods. The excipients used may be, for example, (1) inert diluentssuch as calcium carbonate, lactose, calcium phosphate, sodium phosphate,and the like; (2) granulating and disintegrating agents such as cornstarch, potato starch, alginic acid, and the like; (3) binding agentssuch as gum tragacanth, corn starch, gelatin, acacia, and the like; and(4) lubricating agents such as magnesium stearate, stearic acid, talc,and the like. The tablets may be uncoated or they may be coated by knowntechniques to delay disintegration and absorption in thegastrointestinal tract, thereby providing sustained action over a longerperiod. For example, a time delay material such as glyceryl monostearateor glyceryl distearate may be employed. They may also be coated by thetechniques described in the U.S. Pat. Nos. 4,256,108; 4,160,452; and4,265,874, to form osmotic therapeutic tablets for controlled release.

In some cases, formulations for oral use may be in the form of hardgelatin capsules wherein the active ingredients are mixed with an inertsolid diluent, for example, calcium carbonate, calcium phosphate,kaolin, or the like. They may also be in the form of soft gelatincapsules wherein the active ingredients are mixed with water or an oilmedium, for example, peanut oil, liquid paraffin, or olive oil.

The pharmaceutical formulations may also be in the form of a sterileinjectable solution or suspension. This suspension may be formulatedaccording to known methods using suitable dispersing or wetting agentsand suspending agents. The sterile injectable preparation may also be asterile injectable solution or suspension in a non-toxicparenterally-acceptable diluent or solvent, for example, as a solutionin 1,4-butanediol. Sterile, fixed oils are conventionally employed as asolvent or suspending medium. For this purpose any bland fixed oil maybe employed including synthetic mono- or diglycerides, fatty acids(including oleic acid), naturally occurring vegetable oils like sesameoil, coconut oil, peanut oil, cottonseed oil, or synthetic fattyvehicles, like ethyl oleate, or the like. Buffers, preservatives,antioxidants, and the like, can be incorporated as required.

Formulations contemplated for use in the practice of the presentinvention may also be administered in the form of suppositories forrectal administration of the active ingredients. These formulations maybe prepared by mixing the active ingredients with a suitablenon-irritating excipient, such as cocoa butter, synthetic glycerideesters of polyethylene glycols (which are solid at ordinarytemperatures, but liquify and/or dissolve in the rectal cavity torelease the active ingredients), and the like.

In accordance with yet another aspect of the invention, there areprovided animal models which are useful to study human response toagents for possible up-regulation of CYP3A. Invention animal modelsinclude transgenic non-human animals (e.g. rodents and the like)transformed with nucleic acid encoding human SXR. Those of skill in theart can readily determine suitable methods for introducing nucleic acidencoding human SXR into a suitable host. In another embodiment of theinvention, transgenic animal models are provided wherein SXR andhomologs thereof (e.g., PXR) have been “knocked out” so as to render theanimal model substantially free of any background activity contributedby SXR or homologs thereof. The resulting transgenic animals arereferred to herein as “knock-out” animals, based on the protocol wherebysequence encoding SXR or homologs thereof is deleted from the genome orsuch sequence in sufficiently disrupted or inactivated so as to precludeexpression of active receptor by the host organism. Those of skill inthe art can readily identify numerous methods whereby deletion orinactivation of target sequence (e.g., SXR or homologs thereof) can beaccomplished.

Recent important advances have been made in the understanding of themechanism through which foreign chemicals impact on the P450-dependentmetabolic processes. One key discovery is the establishment of potentialroles for orphan receptor SXR in mediating the induction of CYP3A familyof P450's in response to a variety of xenochemicals including certaindrugs and steroids. Electrophoretic mobility shift assays reveal thatSXR/RXR heterodimer can bind the IR-6 and DR-3 response elements derivedfrom the promoters of human CYP3A4 genes. Moreover, SXR activates theresponse element-containing synthetic reporter genes in response to somedrug/xenochemicals and steroid hormones, suggesting a potential role ofSXR in CYP3A induction. In accordance with the present invention, it isdemonstrated that SXR can activate CYP3A cellular promoters in primaryrat hepatocyte cultures. Furthermore, introducing the human SXR toproduce a transgenic mouse is sufficient to render the mouse liver witha human profile of CYP3A gene inducibility, and expression of anactivated form of SXR results in specific and constitutive upregulationof CYP3A, establishing a central role of SXR/PXR in CYP3A geneinduction.

In addition to SXR and the mouse homolog, PXR, nuclear receptors CAR,PPARα, FXR and LXRα, have also recently been implicated in theregulation of other CYP family members (for a review, see Waxman 1999).Moreover, endogenous ligands of each of these nuclear receptors havebeen identified and physiological receptor functions are emerging,leading to the proposal that these receptors may primarily serve tomodulate hepatic P450 activity in response to endogenous dietary orhormonal stimuli.

Although there are substantial structural and catalytic similaritiesamong the various members of the CYP3A family across species lines,there are important differences in regulatory control of these genes(for reviews, see Gonzalez, 1990, and Nelson, 1999). For example, aclear discrepancy between human and rodents is that RIF induces CYP3A4in human liver (Watkins et al., 1985) but does not induce its homologuesCYP3A23 in rat (Wrighton et al., 1985) and CYP3A11 in mouse (Schuetz etal., 1996), respectively. Rifampicin does induce CYP3A6, the homologousform in rabbit (Kocarek et al., 1995), yet in the rabbit, PCN, whichinduces CYP3A23 in rat liver (Wrighton et al., 1985), does not induceCYP3A6. PCN is also a poor activator for CYP3A4 (Schuetz et al., 1993,Kocarek et a., 1995, Blumberg et al. 1998, and Lehmann et al., 1998).

Based on two pieces of evidence, it is proposed that SXR/PXR, ratherthan the gene structure, determine the inducibility of CYP3A genes: (1)SXR and PXR share similar DNA binding profiles. Steroid and xenobioticinducibility of human CYP3A4 has been localized to an IR-6 containing19-bp element (Barwick et al. 1996), and a similar element is alsopresent in the rabbit CYP3A6 genes (Barwick et al. 1996); whereas thepromoters of rodent CYP3A genes contain DR-3 elements. Electrophoreticmobility shift assays reveal that both SXR:RXR and PXR:RXR heterodimersbind to DR-3 and IR-6 elements efficiently (Blumberg et al., 1998, andLehmann et al., 1998); (2) When cultured rat hepatocytes weretransfected with vectors bearing DR-3 or IR-6-containing 5′-flankingresponse DNA element from CYP3A23, CYP3A4, or CYP3A6 genes, reportergene activity was induced on treatment with PCN; whereas RIF treatmenthad no effect. When the same vectors were transfected into rabbithepatocytes, increased activity was observed on treatment of the cellswith RIF but not with PCN (Barwick et al. 1996). However, suchtrans-species gene transfer has not been tested in the context of thecellular promoters of the CYP genes.

In accordance with the present invention, it is demonstrated that SXRdictates the inducibility of CYP3A in hepatocyte cultures and intransgenic mice, and the DR-3 and IR-6 response elements areinterchangeable in the context of rat CYP3A23 cellular promoter. Theseresults provide strong evidence that the host cellular environment,SXR/PXR herein, rather than the structure of the gene dictates thepattern of CYP3A inducibility. Furthermore, a system of trans-speciesgene transfer and CYP3A inducibility has been established, which could,in turn, provide a unique technique for identifying mechanisms ofinduction and advancing the development of appropriate toxicologicalmodels for human safety assessment.

Thiazolidinediones (TZDs) are a new class of oral antidiabetic agents.They selectively enhance or partially mimic certain actions of insulin,causing a slowly generated antihyperglycaemic effect in Type II(noninsulin dependent) diabetic patients. To date two TZDs, firsttroglitazone (Rezulin) and more recently Rosiglitazone (BRL49653), havebeen introduced into clinical use. However, hepatotoxicity, which wasanecdotally reported as a problem with ciglitazone and englitazone, hasproved to be the main clinical concern with troglitazone (for a review,see Day, 1999). In clinical trials, troglitazone-induced hepatotoxicity(alanine aminotransferase level>three times the upper limit of normal)was identified in 1.9% of 2510 patients; these abnormalities resolvedwith discontinuation of therapy with the drug (for a review, see Watkinsand Whitcomb, 1998). Indeed, hepatic dysfunction and/or fulminanthepatitis leading to hepatic failure has been reported in patientsreceiving troglitazone (Neuschwander-Tetri et al, 1998, Shibuya et al1998, and for reviews, see Watkins and Whitcomb, 1998, and Day, 1999).However, the mechanism of the liver toxicity by TZDs remains largelyunknown.

In accordance with the present invention, it has been shown that membersof the TZDs selectively activate SXR both in hepatocyte cultures and intransgenic animals. Among the tested TZDs, BRL has the highest bindingaffinity to PPARγ with a Kd of approximately 40 nM (Lehmann et al.,1995), yet fail to activate SXR; whereas troglitazone and ciglitazoneactivate SXR. The activation of SXR and subsequent upregulation of CYP3Agene by troglitazone and ciglitazone, together with the fact thatconstitutive activation of SXR causes liver toxicity, provides apotential mechanism for the known clinical liver toxicity by certainTZDs. However, it remains to be seen whether BRL clinically exhibitsreduced or an absence of liver toxicity. Although the VPSXR-inducedliver toxicity does not completely mimic troglitazone-induced humanliver disease in histologic appearance, it is possible that the acutehepatocellular injury present in transgenic mice is a precursor lesionto the confluent necrosis observed in patients with troglitazone injury.The results presented herein also raise the notion that activation ofSXR and/or upregulation of CYP3A gene may be applied to screen futureTZD drugs and other pharmaceutical compounds. The Alb-SXR transgenicmice, as well as the hepatocyte transfection system, will be invaluabletools in such applications.

The factors responsible for human variation in CYP3A expression areunder intense investigation. This variation is believed to influencedrug response for up to one-third of all drugs and may also contributeto inter-individual differences in health effects resulting fromexposure to CYP3A-metabolized carcinogens in the environment (Kolars etal., 1994). The extent to which drugs, like RIF, can up-regulate CYP3Ais of therapeutic importance because it is coadministered with so manydrugs that are CYP3A substrates and thus contributes to increased ordecreased effectiveness of these drug therapies as well as adverse sideeffects (Borcherding et al., Arch Intern Med 1992;152(11):2348 Update onrifampin drug interactions. II.Borcherding S M, Baciewicz A M, Self TH., and Hebert et al., 1992). However, RIF does not induce CYP3A23 inrat (Wrighton et al., 1985) and CYP3A11 in mouse (Schuetz et al., 1996),respectively, which in turn limits the application of rodent models instudying RIF-mediated CYP induction. In accordance with the presentinvention, Alb-SXR transgenic mice have been successfully generatedwhich are readily responsive to RIF to induce CYP3A gene. The doses ofRIF (1-10 mg/kg) that induce CYP3A in these mice are in the range of thestandard oral dosing regimen in humans (300-600 mg per 70-kg man).Moreover, the dynamics and the reversibility of RIF-mediated CYP3Ainduction in the Alb-SXR mice are in agreement with the observation inhumans (Kolars et al., 1992), indicating the Alb-SXR mice are indeed anexcellent rodent model to study RIF-induced CYP3A response.

The invention will now be described in greater detail by reference tothe following non-limiting examples.

EXAMPLE 1 cDNA Identification

SXR was identified from a human genomic library (Clontech) hybridizedwith a full-length cDNA encoding Xenopus BXR (Blumberg et al., 1998a)under reduced stringency conditions (hybridization in 0.5 M NaPO₄ pH7.0, 7% sodium dodecyl sulfate (SDS), 5% dextran sulfate at 65° C.overnight, washing three times twenty minutes in 2× standard salinecitrate solution (0.15M saline containing 0.015M sodium citrate, pH 7)(SSC), 0.1% SDS at 37° C.). Restriction mapping and Southern blotanalysis showed that three exons were contained within the 9 kb EcoRIhybridizing fragment. This fragment was used to probe a Northern blot ofmultiple types of human tissue (Clontech) at high stringency(hybridization as above, washing twice for 20 minutes in 0.1×SSC, 0.1%SDS at 50° C.) and hybridization was detected in liver. A human livercDNA library (Stratagene, La Jolla, Calif.) was subsequently screenedusing the same conditions, and four independent clones were identified.Each of these clones was sequenced on both strands within the proteincoding region. DNA sequences were compiled and aligned using theprograms of Staden (R. Staden, Nucl. Acids Res. 14:217-231, 1986),University of Wisconsin Genetics Computer Group (Devereaux et al., Nucl.Acids Res. 12:387-395, 1984). Database searching was performed using theBLAST network server at the National Center for BiotechnologyInformation (Altschul et al., J. Mol. Biol. 215:403-410, 1990). PXR wasisolated from a mouse liver cDNA library (Stratagene) by screening withthe SXR protein coding region at reduced stringency (5×SSC, 43%formamide, 5× Denhardts, 0.1% SDS, 0.1 mg/ml denatured, sonicated salmonsperm DNA at 37° C.). Three, twenty minute washes were performed in0.5×SSC, 0.1% SDS at 50° C.

EXAMPLE 2 Ability of SXR to Heterodimerize with RXR

The protein coding region of SXR was PCR amplified and subcloned intoNcoI and BamHI sites of the vector pCDG1 (Blumberg, supra, 1998a) usingExoIII-mediated ligation independent cloning (Li and Evans, Nucl. AcidsRes. 25, 4165-4166, 1997). During this process the putative initiatorLeu was converted to Met with a Kozak consensus sequence CCATGG. Theactual response elements and the number of copies are as follows: thebase vector is tk-luc in all cases (Hollenberg et al., Nature318:635-641, 1985):

DR-1, tk(ApoAI)₄ (Ladias and Karathanasis, Science 251:561-565, 1991);

DR-2, tk(Hox-B1-RARE)₂ (Ogura and Evans, Proc. Natl. Acad. Sci. (USA)92:387-391, 1995);

βDR-3, tk(CYP3A2)₃ (Kliewer et al, Cell 92:73-82, 1998),

βDR-4, tk(MLV-TRE)₂ (Umesono et al., Cell 65:1255-1266, 1991);

βDR-4, tk(LXRE)₃ (Willy et al., Genes Dev. 9:1033-1045, 1995);

βDR-5, tk(βRARE)₃ (Sucov et al., Proc. Natl. Acad. Sci. (U.S.A)87:5392-5396, 1990);

TRE_(p), tk(TRE_(p))₂ (Umesono et al, supra, 1991).

Direct repeat 0-15 (DR-0 up to DR-15) oligonucleotides employed hereinhad the following sequences:

DR-0: (SEQ ID NO:12) catagtc AGGTCA AGGTCA gatcaac; DR-1: (SEQ ID NO:13)catagtc AGGTCA t AGGTCA gatcaac; DR-2: (SEQ ID NO:14) catagtc AGGTCA atAGGTCA gatcaac; DR-3: (SEQ ID NO:15) catagtc AGGTCA tat AGGTCA gatcaac;DR-4: (SEQ ID NO:16) catagtc AGGTCA tata AGGTCA gatcaac; DR-5: (SEQ IDNO:17) catagtc AGGTCA tatat AGGTCA gatcaac; DR-6: (SEQ ID NO:18) catagtcAGGTCA tatata AGGTCA agatcaac; DR-7: (SEQ ID NO:19) catagtc AGGTCAtatatat AGGTCA gatcaac; DR-10: (SEQ ID NO:20) catagtc AGGTCA tatatatataAGGTCA gatcaac; DR-15: (SEQ ID NO:21) catagtc AGGTCA tagtagtagtagtagAGGTCA gatcaac.GAL4-SXR was constructed by subcloning aa 107-434 of SEQ ID NO:2 intopCMX-GAL4 (Perlmann, supra, 1993).

Similarly, the PXR.1 protein coding region was PCR amplified andsubcloned into a NcoI-BamHI cut in pCDG1, while amino acids 104 to 431were subcloned into CMX-GAL4. Reporter plasmids were constructed bysynthesizing three-copy response elements and subcloning into aHindIII-BamHI cut in pTk-luc (Hollenberg et al., Cell 49:39-46, 1987).

CV-1 cells were maintained in Dulbecco's Modified Eagle's Medicine(DMEM) containing 10% resin-charcoal stripped calf bovine serum (CBS).Liposome-mediated transient transfections were performed using1,2-bis(oleoyloxy)-3-(trimethylammonio)propane (DOTAP) reagent(Boehringer Manheim) at a concentration of 5 μg/ml in DMEM containing10% resin charcoal stripped fetal bovine serum in 96-well format using aBeckman Biomek 1000 laboratory workstation as described in (Blumberg etal., Proc. Natl. Acad. Sci. (USA) 93:4873-4878, 1996). Test ligands wereadded the next day in DMEM containing 10% delipidated fetal bovine serum(FBS). After 18-24 hours incubation, the cells were lysed and luciferasereporter gene assays and β-galactosidase transfection control assayswere performed as described in (Blumberg, supra, 1996). Reporter geneexpression was normalized to the β-galactosidase transfection controland expressed as relative light units per optical density unit perminute of β-galactosidase activity, or fold induction over solventcontrol. Each data point represents the average of triplicateexperiments +/−standard error and was replicated in independentexperiments.

EXAMPLE 3 Cell Culture and Transfection Studies

To determine whether the activity of SXR was ligand-dependent, mixturesof natural and synthetic compounds were tested for their ability toactivate SXR in transfection-based assays. Thus, the protein codingregion of SXR was PCR amplified and subcloned into NcoI and BamH1 sitesof the vector pCDG1 (see Blumberg et al., supra). During this processthe putative initiator Leu was converted to Met with a Kozak consensussequence CCATGG.

GAL4-SXR was constructed by cloning amino acid residues 134-446 of SXRinto pCMX-GAL4 (see Perlman et al. supra). CV-1 cells were maintained inDMEM containing 10% resin-charcoal stripped calf bovine serum.Liposome-mediated transient transfections were performed using DOTAPreagent (Boehringer Manheim) at a concentration of 5 mg/ml in DMEMcontaining 10% resin charcoal stripped fetal bovine serum in 96-wellformat using a Beckman Biomek 1000 laboratory workstation as previouslydescribed by Blumberg et al., in Proc. Natl. Acad. Sci. (USA) 93:4873(1996)).

Ligands were added the next day in DMEM containing 10% delipidated FBS.After 18-24 hours incubation, the cells were lysed and luciferasereporter gene assays and b-galactosidase transfection control assaysperformed as previously described by Blumberg et al. (1996), supra.Reporter gene expression was normalized to the β-galactosidasetransfection control and expressed as relative light units per O.D. perminute of β-galactosidase activity or fold induction over solventcontrol. Each data point (see FIG. 2) represents the average oftriplicate experiments +/−standard error and was replicated inindependent experiments.

EXAMPLE 4 DNA-binding Analysis

Electrophoretic mobility shift assays were performed using in vitrotranscribed, translated proteins (TNT, Promega). Proteins (1 μl each)were incubated for 20 minutes at room temperature with 100,000 cpm ofKlenow-labeled probes in 10 mM Tris pH 8, 100 mM KCl, 6% glycerol, 0.05%NP-40, 1 mM dithiothreitol (DTT), 100 ng/μl poly dI:dC (Pharmacia,Piscataway, N.J.) and then electrophoresed through a 5% polyacrylamidegel in 0.5×TBE (45 mM Tris-base, 45 mM boric acid, 1 mMethylenediaminetetraacetic acid (EDTA) at room temperature. Forcompetition binding, protein plus unlabeled oligonucleotides at five orfifty fold molar excess were preincubated for ten minutes on ice, thenlabeled probes were added and incubated for 20 minutes at roomtemperature. Electrophoresis was as above. The IR seriesoligonucleotides tested had the following sequences:

IR-0, agcttAGGTCATGACCTa; (SEQ ID NO:25) IR-1, agcttAGGTCAgTGACCTa; (SEQID NO:26) IR-2, agcttAGGTCAcgTGACCTa; (SEQ ID NO:27) IR-3,agcttAGGTCAcagTGACCTa, (SEQ ID NO:28 IR-4, agcttAGGTCAcatgTGACCTa; (SEQID NO:29) IR-5, agcttAGGTCAcactgTGACCTa; (SEQ ID NO:30) IR-6,agcttTGAACTcaaaggAGGTCA); and (SEQ ID NO:31) IR-M, agcttACGTCATGACGTa.(SEQ ID NO:32)

Mutations in the IR-M nucleotide sequence prevented binding of theheterodimer to the response element.

CYP3A oligonucleotides tested had the following sequences: CYP3A4, (SEQID NO:33) tagaataTGAACTcaaaggAGGTCAgtgagtgg; CYP3A5, (SEQ ID NO:34)tagaataTGAACTcaaaggAGGTAAgcaaaggg; and CYP3A7, (SEQ ID NO:35)tagaataTTAACTcaatggAGGCAgtgagtgg

EXAMPLE 5 Plasmid Constructs and Mutagenesis

The CYP3A23 cellular promoter reporter, PGL3-CYP3A23, was cloned byinserting the PCR-amplified 5′ regulatory sequence of rat CYP3A23 gene(nt −1360 to 82) (Burger, et al. 1992) into the PGL3 vector (Promega).PGL3-CYP3A4 contains up to nt −1093 of the 5′ flanking regions of thehuman CYP3A4 gene (Hashimoto et al., 1993). Site-directed mutagenesiswas performed by the PCR overextension method (Ho et al., 1989). ThePCR-amplified sequences and target mutagenesis were confirmed by DNAsequencing.

The expression vectors for the wild type SXR (pCDG-HX7), an activatedform of SXR (pVPG-HX7), and the wild type PXR (pCDG-PXR) were describedpreviously (Blumberg et al, 1998).

EXAMPLE 6 Preparation of Hepatomytes, DNA Transfections and DrugTreatment

Primary cultures of rat hepatocytes were prepared as describedpreviously (Li et al, 1991, and Barwick, et al. 1996). Lipofectin(Gibco-BRL)-mediated DNA transfections were carried out as described(Barwick, et al. 1996). When necessary, cell were treated with RIF, DEX,PCN, nifedipine, CTZ, corticosterone, coumestrol, RU486, cortisol,17β-estrodiol (E2), pregnenolone, progesterone, cortisone (10 μM each),phenobarbital, 3-methylcholanthrene (3MC) (2 mM each), or the controlsolvent. All compounds were purchased from Sigma.

EXAMPLE 7 SXR Imparts Trans-species Drug Response of CYP3A Genes to RatHepatocyte Cultures

A panel of natural and synthetic steroid and nonsteroid compounds weretested for their ability to activate SXR and/or PXR intransfection-based assays using primary rat hepatocytes as recipientcells and the cellular promoters of the rat CYP3A23 gene or the humanCYP3A4 gene as reporters. In the absence of SXR, the most potent andefficacious tested activators for CYP3A23 werepregnenolone-16-carbonitrile (PCN), nifedipine, RU486 (anotherantiglucocorticoid), whereas rifampicin (RIF), clotrimazole (CTZ),phenobarbital, 3-methylcholanthrene (3MC, a known CYP1A2 activator),corticosterone, coumestrol, cortisol, E2, progesterone pregnenolone, andcortisone fail to activate or behave as poor activators (FIG. 8A). Thisprofile of activation reflects the responsiveness of the endogenous PXR,a rodent homologue of SXR. The failure of RIF to induce rat CYP3A23 geneis consistent with previous observation (Wrighton et al., 1985, andSchuetz et al., 1996). With the co-transfection of SXR, significantinduction of CYP3A23 was achieved by RIF, CTZ, phenobarbital, E2, andpregnenolone. The induction of CYP3A23 by nifedipine, and RU486 alsoincreased significantly; while the activation of CYP3A23 by PCN remainedunchanged in the presence of SXR (FIG. 8A). Therefore, transfection ofSXR render the responsiveness of rat CYP3A gene by RIF, a known humanspecific CYP3A activator.

When the human CYP3A4 cellular promoter was used as the reporter, asimilar response profile was observed, except that E2 did not induceCYP3A4, and nifedipine did not further potentiate CYP3A4 induction inthe presence of SXR (FIG. 8B). Thus, the human CYP3A4 can be activatedby the rodent-specific activator PCN when the promoter was introducedinto the rodent cellular environment, presumably via the activation ofthe endogenous PXR; on the other hand, RIF can active the CYP3A4 in therodent cellular environment with the introduction of human SXR. TheSXR-mediated activation of CYP3A23 or CYP3A4 cellular promoter by RIFexhibited dose dependence of both receptor and ligand (data not shown).

The fact that SXR is necessary and sufficient to render the induction ofboth human CYP3A4 and rat CYP3A23 gene in rodent hepatocytes by RIFsuggested that the host cellular environment, SXR/PXR herein, ratherthan the gene structure, dictates the patterns of inducibility of CYP3Agenes. The above notion would predict: (1) The SXR/PXR response elementis essential for the activation of CYP3A genes; and (2) The responseelements of SXR and PXR are interchangeable. Therefore, mutagenesisanalysis was performed on the promoter of the rat CYP3A23 gene toexamine these predictions. In vitro electrophoretic mobility shiftassays showed that both SXR:RXR and PXR:RXR heterodimers efficientlybind to the DR-3 element (5′ TGAACTtcaTGAACT 3′) (SEQ ID NO: 39) in theCYP3A23 promoter (Blumberg et al., 1998, 1998). As shown in FIG. 8C,mutation of both half sites (DR3/M1) or a single half site (DR3/M2)abolished the PXR and/or SXR-mediated activation by PCN, RIF, and CTZ;On the other hand, replacement of the wild type DR-3 element by an IR-6element of the human CYP3A4 gene promoter (Blumberg et al., 1998, andKliewer et al., 1998) successfully rescueS the inducibility by PCN, RIFand CTZ.

Taken together, the transfection results demonstrate that nuclearreceptors SXR/PXR are essential in determining patterns of CYP3Ainducibility. In addition, these results establish successfuldevelopment of a cell culture system allowing trans-species genetransfer and CYP3A inducibility.

EXAMPLE 8 Generation and Identification of Transgenic Mice

To generate Alb-SXR and Alb-VPSXR transgenes, the SXR and VPSXR cDNAwere released from pCDG-HX7 and pVPG-HX7 (Blumberg et al., 1998), andcloned into the Bam HI site downstream of the mouse albuminpromoter/enhancer (Pinkert et al., 1987), respectively. A SV40intron/poly (A) sequence (Xie et al., 1999) was subsequently placeddownstream of SXR and VPSXR cDNAs. The 8.45 kb Alb-SXR, and 8.75 kbAlb-VPSXR transgenes were excised from the vector via Not I and Asp 718digestion, and purified from agarose gel using QIAquick Gel ExtractionKit (QIAGEN). Microinjection of transgene into one-cell CB6F1 mousezygotes was carried out at the Salk Institute Transgenic AnimalFacility. All mice were handled in an accredited Institute facility inaccordance with the institutional animal care policies.

Genomic DNA was isolated as described before (Xie et al. 1999). Thepolymerase chain reaction (PCR) was used to screen the transgenepositive mice. Two oligonucleotides used to screen Alb-SXR mice are5′-GAGCAATTCGCCATTACTCTGAAGT-3′ (SEQ ID NO: 36) (annealing to SXR cDNA),and 5′-GTCCTTGGGGTCTTCTACCTTTCTC-3′ (SEQ ID NO: 37) (annealing to theSV40 sequence downstream of the transgene in the transgene cassette).Another two oligonucleotides used to screen Alb-VPSXR are5′-GACGATTTGGATCTGGACATGTTGG-3′ (SEQ ID NO: 38) (annealing to VP16sequences), and 5′-GTTTTCATCTGAGCGTCCATCAGCT-3′ (SEQ ID NO: 40)(annealing to the SXR cDNA). PCR was carried out in a DNA thermal cycler(Perkin-Elmer/Cetus) using the following program: 94° C. for 1 min, 58°C. for 2 min, and 72° C. for 3 min and products were analyzed byelectrophoresis on a 1% agarose gel. The transgene integration statuswas analyzed by Southern blot using transgene specific probes asdescribed before (Xie et al. 1999).

EXAMPLE 9 Generation of Alb-SXR and Alb-VPSXR Transgenic Mice

Transgenic mice expressing wild-type or an activated form of SXR underthe control of the liver-specific promoter/enhancer for the mousealbumin gene (Pinkert et al., 1987) were generated by injection ofone-cell CB6F1 mouse zygotes with the transgene diagramed in FIG. 2A.This promoter fragment has been shown to direct faithfully theexpression of the transgene in the liver of transgenic mice (Pinkert etal., 1987). The activated form of SXR (VPSXR) was generated by fusingthe VP16 activation domain of the herpes simplex virus to theamino-terminal of SXR. Transfection of VPSXR expression vector into rathepatocytes resulted in constitutive upregulation of the CYP3A23 gene(data not shown). Transgene-positive founders were identified by PCRusing a pair of transgene-specific oligonucleotides, and the integrityof both transgenes was confirmed by Southern blot analysis (data notshown). A total of two and seven gene-positive founders were obtainedfor Alb-SXR and Alb-VPSXR transgene, respectively.

The expression of transgenes was assessed by Northern blot analysis ofRNA from the mouse livers using a transgene-specific probe. Thus, twentymicrogram of liver total RNAs were subjected to Northern blot analysis.The membranes were hybridized with [³²P]-labeled 3 kb SXR-SV40 DNAfragment from the transgene. The filters were subsequently stripped andreprobed with PXR cDNA probe, and the glyceraldehyde-3-phosphatedehydrogenase (GAPDH) cDNA for the purpose of loading control. Thetransgene transcripts (2.6 kb and 2.9 kb for Alb-SXR and Alb-VPSXRtransgene, respectively) were detected in the liver of Alb-SXR, andAlb-VPSXR transgenics, but not in a nontransgenic control animal. Theexpression of endogenous PXR remains unchanged in the transgenic mice.

Initial Northern blotting revealed that Alb-SXR line 2198, and Alb-VPSXRlines 2224 and 2218 had relatively high expression of the transgenes(data not shown), and were characterized further. The expression ofAlb-SXR (2.6 kb) or Alb-VPSXR (2.9 kb) transgene was specificallydetected in the livers of transgenic mice but not in their nontransgeniclittermates. Furthermore, the expression of SXR transgenes did not alterthe expression of endogenous PXR. No transgene expression was seen inthe small intestine, brain and kidney, consistent with thetissue-specificity of the albumin promoter (Pinkert et al., 1987).

EXAMPLE 10 Drug Responsiveness of CYP3A in SXR Transgenic Mice

The animals were allowed free access to food and water at all times. RIF(1-10 mg/kg when necessary), BRL (20 mg/kg, a gift from Dr. RichardHeyman of Ligand Pharmaceutical), ciglitazone (150 mg/kg, Biomol), andtroglitazone (150 mg/kg) were administered via gastric gavage. Whennecessary, mice were treated with a single intraperitoneal injection ofDEX (50 mg/kg), PCN (40 mg/kg), or CTZ (50 mg/kg).

To examine the drug response of the endogenous liver CYP3A11 gene,animals were treated with a single dose of compounds 24 h beforesacrifice, and the CYP3A11 gene expression was evaluated by Northernblot analysis on liver total RNA. Total RNA was prepared from tissuesusing the TRIZOL Reagent (Gibco-BRL). RNA was separated on 1.25%agarose-6% formaldehyde gel and transferred to a Nytran membrane(Schleicher & Schuell). To detect specific transcripts, [³²P]-cDNAprobes labeled by Random Primer Labeling Kit (Boringher) were hybridizedto the membranes. The probe used to detect transgene contains both theSXR cDNA and the SV40 sequences. The PXR cDNA probe was as describedpreviously (Blumberg, et al. 1998). The probes of CYP3A11 gene (nt 1065to 1569) (Yanagimoto et al 1992), CYP7A (nt 973 to 1453) (Jelinek etal., 1990), CYP1A2 (nt 1151 to 1565) (Kimura et al., 1984) were clonedby RT-PCR using mRNA from wild type mouse liver. The filters weresubsequently stripped and rehybridized with a murineglyceraldehyde-3-phosphate dehydrogenase (GAPDH) cDNA probe for RNAloading normalization.

As observed by Northern blot analysis, RIF (5 mg/kg body weight)specifically induced the expression of CYP3A11 in transgenic mice butnot in their wild type littermates. Alb-SXR transgenic mice or controlnontransgenic animals were treated with a single dose of RIF (5 mg/kg,gastric gavage), CTZ or PCN (50 mg/kg and 40 mg/kg, respectively,intraperitoneal injection). Tissues were harvested 24 h later andsubjected to Northern blot analysis. Membranes were probed with CYP3A11cDNA probe, and were subsequently stripped and reprobed with GAPDH andtransgene specific probes. The increased expression of CYP3A11 intransgenics in response to RIF is of particular note.

The inability of RIF to induce CYP3A11 in wild type mice at this dose isconsistent with previous observations (Schuetz et al., 1996). Inagreement with transfection results, CTZ caused a moderate level, and ahigher level of CYP3A11 induction in wild type animals and Alb-SXR mice,respectively; PCN is an equally efficacious CYP3A11 inducer in both wildtype and transgenic animals. The induction of CYP3A11 in Alb-SXR mice isligand dependent, as no CYP3A11 induction was observed in the absence ofan inducer, and the level of transgene expression remained unchangedupon CYP3A11 gene activation.

Dynamics and dose-response of RIF treatment was investigated in Alb-SXRtransgenic mice. In the study of dynamics, mice were subjected to dailytreatment of RIF for the indicated period of time, and tissues wereharvested 24 h after the last treatment. In the study of dose-response,mice were treated with a single dose of indicated amounts of RIF 24 hbefore tissue harvest. The reduction of RIF-induced expression ofCYP3A11 by five days of RIF withdrawal after an initial 7-day treatmentis significant. The CYP3A11 induction by RIF is rapid, and a significantinduction was achieved after 12 h of RIF administration, with a plateauachieved by 24 h in the continuous presence of RIF. No CYP3A11 inductionwas observed in nontransgenic mice even after 7 d of RIF administration.Moreover, the RIF-induced expression of CYP3A11 was reversible,significant reduction of CYP3A11 expression was seen by five days of RIFwithdrawal after an initial 7-day treatment (lane 7). The RIF-mediatedCYP3A11 induction is also dose-dependent, increased hepatic CYP3A11 mRNAwas seen with as little as 1 mg/kg of RIF administration, and theinduction was further enhanced with increasing does of RIF, plateauingaround 3-5 mg/kg. The dynamics and the reversibility of CYP3A inductionby RIF is in agreement with the observation in humans (Kolars et al.,1992).

The CYP3A11 gene is constitutively induced in the livers of Alb-VPSXRtransgenic mice, and its expression was not further enhanced by RIFtreatment. Of note, the upregulation of CYP gene is liver- andCYP3A11-specific, as: (1) the expression of CYP3A11 in the smallintestine remains unchanged, (2) the expression of liver CYP7A gene(cholesterol 7α-hydroxylase), as well as the liver-specific CYP1A2 gene,remains unchanged in the Alb-VPSXR mice. CYP7A is a key enzyme of bileacid biosynthesis, and a responsive gene of FXR (Forman, et al., 1995,Wang et al, 1999, Park et al., 1999, and Makishima et al., 1999).

EXAMPLE 11 Selective Activation of SXR, but Not PXR, by Members of theThiazolidinedione (TZD) Family of PPARγ Ligands

To examine whether TZDs activate SXR and/or PXR, rat hepatocytes weretransiently transfected with CYP3A23 promoter reporter alone, ortogether with expression vectors for SXR or PXR. The transfected cellswere subsequently treated with a panel of natural PPARγ ligand, orsynthetic TZDs. The CYP3A23 gene was not activated by tested PPARγligands in the presence of endogenous PXR (FIG. 10A), or withcotransfection of PXR in addition to endogenous proteins (data notshown). On the contrary, in the presence of SXR, while the natural PPARγligand 15d-PGJ2 and the synthetic BRL49653 fail to activate, two othersynthetic TZDs, troglitazone and ciglitazone, activated CYP3A gene bythree folds and eight folds, respectively. As controls, WY 14643, aPPARγ-specific ligand, and LY171883, a weak pan-activator for PPARs,fail to activate SXR.

The transfection results were further substantiated by in vivoactivation assay. Liver RNAs were harvested 24 h after single dose ofTZD treatment via gastric gavage, and subjected to Northern blotanalysis. While BRL49653 (20 mg/kg) failed to activate CYP3A gene,troglitazone (150 mg/kg) and ciglitazone (150 mg/kg) selectivelyup-regulate the expression of CYP3A11 gene in Alb-SXR transgenic micebut not in their nontransgenic littermates. Ciglitazone is a moreefficacious CYP3A inducer than troglitazone when administered at samedoses, consistent with the hepatocyte transfection results. Incomparison with another known SXR activator/CYP3A inducer, treatment ofciglitazone at 150 mg/kg achieved comparable level of CYP3A induction asRIF at 5 mg/kg.

Taken together, the transfection and animal results demonstrate that twoTZDs, troglitazone and ciglitazone, are selective activators for humanSXR. The SXR-mediated CYP3A gene activation by TZDs, together with thefact that constitutive activation of SXR causes liver toxicity (seebelow), provides a potential mechanism for the known clinical livertoxicity by certain TZDs.

EXAMPLE 12 Constitutive Activation of SXR Results in General GrowthRetardation and Liver Toxicity

The Alb-VPSXR mice exhibit growth retardation, smaller body size andlower body weight in both sexes were well notable at three weeks duringtail biopsy. Shown in FIG. 11 is the growth curve of Alb-VPSXR males ascompared to age- and litter size-matched wild type animals or theirAlb-SXR counterparts. The growth retardation of the Alb-VPSXR mice ismost apparent at 4-5 weeks of age, with a decrease of about 20% in bodyweight compared to wild type or Alb-SXR mice. This percentage decreasedto about 10% by 8-9 weeks, and persisted thereafter (FIG. 11). A similarpattern of growth retardation was also observed in female transgenics(data not shown). The growth retardation may attribute to liver toxicityas described below. No significant body weight changes were seen in theAlb-SXR mice (FIG. 11), indicating the growth retardation is resultedfrom constitutive activation of SXR and upregulation of CYP3A gene inmouse liver.

Autopsy revealed hepatomegaly in the Alb-VPSXR mice. The liver weight of3.5-week-old Alb-VPSXR males increased by 56% when measured aspercentages of body weight (data not shown). The hepatomegaly progressedwith age, and by 2.5 months, the liver accounts for 4.95% of total bodyweight in nontransgenic males; while this percentage is 8.86%, anincrease of 79%, in Alb-VPSXR mice (line 2198) (FIG. 12). Indeed, inspite of their lower body weight as described above, the Alb-VPSXR micehad higher absolute liver weight compared to wild type animals. All fourliver lobes, the large median lobe, the left lateral lobe, the rightlateral lobe, and a caudal lobe, were proportionally enlarged.Macroscopically, the enlarged liver from Alb-VPSXR mice exhibited“nutmeg” features, clinically normally seen as a result of chronicpassive congestion of backflow due to heart disease. The hepatomegaly isliver- and Alb-VPSXR transgene-specific, as (1) No significant changesin organ weight and gross appearance were seen in transgenenon-expressing organs such as the kidney (FIG. 12), and spleen (data notshown); (2) No liver weight changes were seen in untreated Alb-SXR mice(FIG. 12).

Histologic examination of 2.5 month old Alb-VPSXR transgenics revealedremarkabe differences from their littermates. There is markedmicrovesicular steatosis which is most pronounced in zone 3 of the liveracinus (around the central veins). There is also substantial nuclearvariability with enlarged hepatocyte nuclei, especially in zone 3. Largepinkish vacuoles are seen in many hepatocytes adjacent to the nucleus.These appear to be protein accumulations in the perinuclear Golgi andare similar in appearance to those seen in patients with defects inalpha1-antitrypsin that impair its normal intracellular trafficking.These are distributed across the acinus. Additionally, there are foci ofnecrotic hepatocytes (pale pink areas) invaded by neutrophils. Gomori'sTrichrome stains revealed no significant fibrosis in transgenic livers.However, the protein plugs in the hepatocytes of the transgenics areeither very blue or very red, strikingly different from nontransgenicanimals. Therefore, there appears to be accumulation of more than onetype of proteins as intracellular inclusions.

BrdU labeling and immunostaining was performed to examine theproliferation of hepatocytes in transgenics. Four-week-old wild type andAlb-VPSXR transgenic males were injected intraperitoneally with BrdU andparaffin sections of the livers were prepared for immunostaining with ananti-BrdU antibody. 0.5-1% of the transgenic hepatocyte nuclei arepositive for BrdU, and dividing binuclei hepatocytes are notable;whereas the labeled cell is a rare event in their nontransgeniclittermates. Consistent patterns of BrdU labeling were observed inmultiple animals.

Similar general growth retardation, hepatomegaly and liver histologicchanges were also observed in line 2418, another Alb-VPSXR trangenicline with similar levels of transgene expression and constitutiveupregulation of CYP3A11 (data not shown), indicating that the observedphenotypic exhibition is a transgene-specific, rather than anintegration-specific event. As controls, no histologic changes were seenin kidney and small intestines (data not shown).

EXAMPLE 13 Histologic Evaluation BrdU Labeling and Immunohistochemistry

Gross and microscopic evaluation were performed. Tissues were fixed in4% formaldehyde in 1×PBS, embedded in paraffin, sectioned at 5 μm.Hematoxylin and eosin stains, or the Gomori's trichrome stains wereperformed for histological examination. In vivo BrdU labeling wasperformed by intraperitoneal injection of BrdU (Sigma) as described (Xieet al., 1998). The sections were immunostained with a rat monoclonalanti-BrdU antibody MSA250P (1:200) (Accurate) using Vectastain Elite ABCKit (Vector). The chromogen is 3,3′-diaminobenzidine tetrahydrochloride(DAB), and sections were counterstained with Gill's Hematoxylin(Vector).

It will be apparent to those skilled in the art that various changes maybe made in the invention without departing from the spirit and scopethereof, and therefore, the invention encompasses embodiments inaddition to those specifically disclosed in the specification, but onlyas indicated in the appended claims.

1. An expression system comprising: at least one SXR response elementoperably linked to at least one gene, and a nuclear receptor which is amember of the steroid/thyroid hormone superfamily and which responds toxenobiotic compounds and binds to said at least one SXR response elementas a heterodimer with retinoid X receptor (RXR) to activatetranscription of said at least one gene; wherein said at least one SXRresponse element comprises a direct or inverted repeat response elementcomprising at least two half sites RGBNNM separated by a spacer of 0 upto 15 nucleotides  wherein: R is selected from A or G; B is selectedfrom G, C, or T; each N is independently selected from A, T, C, or G;and M is selected from A or C; with the proviso that at least 4nucleotides of said -RGBNNM-sequence are identical with the nucleotidesat corresponding positions of the sequence AGTTCA.
 2. The expressionsystem of claim 1, wherein said nuclear receptor is a steroid xenobioticreceptor.
 3. The expression system of claim 1, wherein said nuclearreceptor is a pregnane X receptor.
 4. The expression system of claim 1,wherein said gene encodes a cytokine, a hormone, a blood component,therapeutic gene, or a toxic protein.
 5. The expression system of claim1, wherein said xenobiotic compound is digitoxin, indomethacin,pregnelone-16-carbonitrile (PCN), tamoxifen, ralozifene, vitamin K,nifedipine, a barbituate or a steroid.
 6. An expression systemcomprising: at least one SXR response element operably linked to atleast one gene, and an expression vector comprising nucleic acidencoding a receptor which is a member of the steroid/thyroid hormonesuperfamily and which responds to xenobiotic compounds and binds to saidat least one SXR response element as a heterodimer with retinoid Xreceptor (RXR) to activate transcription of said at least one gene;wherein said at least one SXR response element comprises a direct orinverted repeat response element comprising at least two half sitesRGBNNM separated by a spacer of 0 up to 15 nucleotides  wherein R isselected from A or G; B is selected from G, C, or T; each N isindependently selected from A, T, C, or G; and M is selected from A orC; with the proviso that at least 4 nucleotides of said -RGBNNM-sequenceare identical with the nucleotides at corresponding positions of thesequence AGTTCA.
 7. The expression system of claim 6, wherein saidnucleic acid encodes a steroid xenobiotic receptor.
 8. The expressionsystem of claim 6, wherein said nucleic acid encodes a pregnane Xreceptor.
 9. The expression system of claim 6, wherein said expressionvector constitutively expresses said nucleic acid.
 10. The expressionsystem of claim 6, wherein said expression vector inducibly expressessaid nucleic acid.
 11. A method for the production of a target proteinin a cell, said method comprising administering to a cell at least onexenobiotic compound, wherein said cell contains: a nucleic acidcomprising at least one SXR response element operably linked to at leastone gene encoding said target protein, and a receptor which is a memberof the steroid/thyroid hormone superfamily and which responds toxenobiotic compounds and binds to said at least one SXR response elementas a heterodimer with retinoid X receptor (RXR) to activatetranscription of said at least one gene; and wherein said at least oneSXR response element comprises a direct or inverted repeat responseelement comprising at least two half sites RGBNNM separated by a spacerof 0 up to 15 nucleotides  wherein: R is selected from A or G; B isselected from G, C, or T; each N is independently selected from A, T, C,or G; and M is selected from A or C; with the proviso that at least 4nucleotides of said -RGBNNM-sequence are identical with the nucleotidesat corresponding positions of the sequence AGTTCA.
 12. The method ofclaim 11, wherein said receptor is a steroid xenobiotic receptor. 13.The method of claim 11, wherein said receptor is a pregnane X receptor.14. The method of claim 11, wherein said xenobiotic compound isdigitoxin, indomethacin, pregnelone-16-carbonitrile (PCN), tamoxifen,ralozifene, vitamin K, nifedipine, a barbituate or a steroid.
 15. Themethod of claim 11, wherein said receptor is provided by expression froma nucleic acid construct encoding same.
 16. A method for the productionof a target protein in a cell, said method comprising administering to acell at least one xenobiotic compound and a nucleic acid comprising atleast one SXR response element operably linked to at least one geneencoding said target protein, wherein said cell contains a receptorwhich is a member of the steroid/thyroid hormone superfamily and whichresponds to xenobiotic compounds and binds to said at least one SXRresponse element as a heterodimer with retinoid X receptor (RXR) toactivate transcription of said at least one gene; and wherein said atleast one SXR response element comprises a direct or inverted repeatresponse element comprising at least two half sites RGBNNM separated bya spacer of 0 up to 15 nucleotides  wherein: R is selected from A or G;B is selected from G, C, or T; each N is independently selected from A,T, C, or G; and M is selected from A or C; with the proviso that atleast 4 nucleotides of said -RGBNNM-sequence are identical with thenucleotides at corresponding positions of the sequence AGTTCA.
 17. Themethod of claim 16, wherein said receptor is a steroid xenobioticreceptor.
 18. The method of claim 16, wherein said receptor is apregnane X receptor.
 19. The method of claim 16, wherein said xenobioticcompound is digitoxin, indomethacin, pregnelone-16-carbonitrile (PCN),tamoxifen, ralozifene, vitamin K, nifedipine, a barbituate or a steroid.20. The method of claim 16, wherein said receptor is provided byexpression from a nucleic acid construct encoding same.
 21. A method forthe production of a target protein in a cell, said method comprisingadministering to a cell at least one xenobiotic compound, and a receptorwhich is a member of the steroid/thyroid hormone superfamily and whichresponds to xenobiotic compounds and binds to at least one SXR responseelement as a heterodimer with retinoid X receptor (RXR) to activatetranscription of at least one gene operably linked to said at least oneSXR response element, wherein said cell contains a nucleic acidcomprising said at least one SXR response element operably linked to atleast one gene encoding said target protein; and wherein said at leastone SXR response element comprises a direct or inverted repeat responseelement comprising at least two half sites RGBNNM separated by a spacerof 0 up to 15 nucleotides  wherein: R is selected from A or G; B isselected from G, C, or T; each N is independently selected from A, T, C,or G; and M is selected from A or C; with the proviso that at least 4nucleotides of said -RGBNNM-sequence are identical with the nucleotidesat corresponding positions of the sequence AGTTCA.
 22. The method ofclaim 21, wherein said receptor is a steroid xenobiotic receptor. 23.The method of claim 21, wherein said receptor is a pregnane X receptor.24. A method for the production of a target protein in a cell, saidmethod comprising inducing synthesis in said cell of a receptor which isa member of the steroid/thyroid hormone superfamily and which respondsto xenobiotic compounds and binds to at least one SXR response elementas a heterodimer with retinoid X receptor (RXR) to activatetranscription of at least one gene operably linked to said at least oneSXR response element, wherein said cell contains: an expression vectorcomprising nucleic acid encoding said receptor operatively associatedwith an inducible promoter, a nucleic acid comprising said at least oneSXR response element operably linked to at least one gene encoding saidtarget protein, and at least one xenobiotic compound; and wherein saidat least one SXR response element comprises a direct or inverted repeatresponse element comprising at least two half sites RGBNNM separated bya spacer of 0 up to 15 nucleotides  wherein R is selected from A or G; Bis selected from G, C, or T; each N is independently selected from A, T,C, or G; and M is selected from A or C; with the proviso that at least 4nucleotides of said -RGBNNM-sequence are identical with the nucleotidesat corresponding positions of the sequence AGTTCA.
 25. The method ofclaim 24, wherein said receptor is a steroid xenobiotic receptor. 26.The method of claim 24, wherein said receptor is a pregnane X receptor.